What Color is a Brachiosaurus?
The Brachiosaurus dinosaur group, if you don't know, are the tallest dinosaurs (if you count his relative, Ultrasaurus in that group), and I've already done a commentary on the idea that they may have a trunk like an elephant.  
            See Article.                                    See Pictures.
But while I was researching the trunk theory, another curious thought struct me. I began to suspect all traditional descriptions of their color might be wrong. Every textbook that describes it or illustrates it probably paints it gray or grayish green, and probably describes it as follows (loosely paraphrased): "The Brachiosaurus is a giant land herbivore, and so it is most likely a solid, grayish monotone like the two giant land herbivores of the present day, the elephant and the rhinoceros."
How can you go wrong with such simple, comparative wisdom. It made sense to me when I first heard it as a kid. It made sense for years after I grew up. It is so wonderfully easy to believe in, so uncontroversial, so effortlessly logical, why on earth would I want to rock the boat and question it? Because now I believe it's wrong.  It's a superficial comparison that makes sense at a superficial level, but my mind wandered to a deeper level of comparison and it didn't hold up logically.
Many fine paleontologists who have endorsed or abide by the "grayish monotone" view, and they are, indeed, fine paleontologists who make valued and wise contributions to the field of paleontology. They didn't see why they were wrong because they were busy looking at other research questions and one brain can only spread itself around so much. If they didn't think this one through better, I'm sure it was because they were thinking other important things through.
This question illustrates what I like to call "Layers of Truth". In some issues, the facts are layered in strata, like sedimentary rock formations. And at each layer, you can find what appears, in good faith, to be true. But if you look to the deeper level, new facts will be revealed that may completely reverse your opinion. And for that reason, I am so vocal in my belief that we need to encourage dialogs and openly entertain alternate views. We can better understand nature by seeing a given question or idea from several points of view. And for the question of the color of the Brachiosaurus, I suspect everybody looked at the question from the exact same point of view, and from the same depth or distance. Thus, everybody saw the same logic and they simply stopped thinking about it and went on to other issues or questions.
A Learning Lesson
This question provides an outstanding illustration of why we should think deeper about even the seemingly obvious ideas, and why we should be willing to listen to new ideas from any source.
Why is the Brachiosaurus NOT like the elephant and rhino in color?
Let's begin with the color of elephants and rhinos. Yes, they are both giant ground dwelling herbivores. They are much bigger than the king predator of their niche, the lion. So they can be grayish because they don't need camouflage colors to hide from the lions. At one level, that makes sense. So you can stop there and think you've found the truth.
The flaw in the logic at that level is the presumption that the design criteria for their skin pigmentation is "Big enough to not need camouflage". But elephants and rhinos are also hairless mammals in a tropical or equatorial savanna environment. And mammals, by definition, are hair-covered. And hair forms a cushion, a protective barrier against the burning radiation of the sun. Why the elephant and rhino lost their hair is not the issue. It's simply a given fact. They did. And because they did, they need to protect their hairless bodies from the sun's radiation. They do so with dust and mud baths. They coat the skin with earth to form an opaque temporary sunscreen.
So, how do you evolve any specialized skin coloration if you are always covering the skin with mud or dust? How can any mutation of color confer any advantage if you won't let the experiment run it's course? In my opinion, the elephant and rhino are both grayish monotones, with no special color adaptations, not because they are land giants, but because they are hairless mammals in a tropical niche and they must protect their naked skin from solar radiation with mud and dust bathes that overwhelm any attempt to evolve skin pigment patterns.
So being a land giant herbivore, bigger than the king predator, is not the design criteria determining need or lack of need for specialized coloration. So comparing the brachiosaurus by this criteria is a moot point. It's not what influences the elephant and rhino coloration, so why should it influence the brachiosaurus color? Now this, by itself, only gets us to the point of saying that the elephant and rhino colors do not support the brachiosaurus assumption of similar color. But neither does it refute the assumption. It simply says, the comparison of brachiosaurus to elephants and rhinos is not necessarily valid. So, at this layer of inspection, all you could argue is brachiosaurs may or may not be grayish, but if so, it is for reasons other than comparison to elephants or rhinos.
The Next Layer
So we need to look at the next layer deeper. We were comparing the brachiosaurus to elephants and rhinos because the criteria for comparison were giant land herbivores. But a brachiosaurus is also a specialized high browser, like the modern giraffe. And lest you choose to drag the elephant back in as a high browser, the true definition of a high browser is one so specialized, it cannot feed from the ground. Elephants can feed easily from the ground, and all the way up to high reaching. But high browsing isn't their exclusive specialization. But it is for the giraffe and it is for the brachiosaurus. So perhaps this is the better comparison.
The giraffe has vivid camouflage coloration, despite the enormous size of adults and the fact that they reportedly can dropkick an adult lion a good country mile. If the adults can easily fight off the king predator, why are the adults camouflaged? Why not just let the kids be spotted, as so many other animal babies are (predator and prey alike) and let the adults be bland monotones? The answer lies in one intriguing drawback to being the true specialized high browser.
The fundamental definition of a specialized high browser is that it feeds where smaller animals cannot reach. And babies and juveniles of it's own species are smaller. So the one drawback of being the specialized high browser is that you are specialized to feed where your own young cannot! It is one of the very few herbivore niches where young and adults cannot feed side by side.
(I don't know of others, but I'm allowing for the possibility other examples do exist).
So, high browsers cannot feed side by side with their own young. And herbivores need to eat for a good part of each day. So the unique body design that allows them to feed in this niche is the same body design that compels their young to feed away from the parents. This means the young are unprotected by the adults  as they feed in detached nursery herds. So obviously, the young should have protective or camouflage color.
So, why do the adults need camouflage color. If the young are hidden by camouflage, aren't they protected? Unfortunately, not for the specialized high browser. The same niche that provides them food requires that they be the biggest, tallest creatures around. And what could be easier to see than the tallest creatures? If a predator is looking for the young, why not just look for the big adults, and then find the young nearby in their unprotected nursery herds. If the adults are easily seen, they will draw predators to the general area, and then the young can be stalked and killed, because they are forced to feed away from the adults (by their specialization of high browsing).
This is why the adult specialized high browser must also be fully camouflage colored. It must be hidden by protective coloration so it doesn't attract the attention of predators to the general location and remind predators that there are un-protected nursery herds feeding nearby. And I propose that the true specialized high browser of the Jurassic, Brachiosaurus, was camouflage-colored for this reason.
So what color might it be?
Now, I don't propose it was colored exactly like the giraffe. The giraffe's color pattern is basically intended to resemble the mottled light and shade of sunlight through the sparse leafy branches of acacia trees. My suggestion for the brachiosaurus would be a leafy greenish body (to look like a bush), a neck matching the color of tall trunks of the tree-like growth it was standing next to and feeding on, and a head with more leafy greenish blotches to match the leafy stuff it was eating. So, to look from a distance, all you would see were short and tall plants. Only if you looked closer and carefully would you see some of the plants were eating each other.
The one weakness of this idea, which I invite my critics to roast me on, is that the premise is predicated on assumptions that young and adults socialized as a herd (a warm-blooded strategy). Trackways of sauropods seem to be supportive of the herd idea, but not conclusive to the brachiosaurs, as far as I know.
But whether I'm right or wrong about the brachiosaurus, the greater issue is that we need to look more carefully at the different layers of comparison when trying to explain the paleo world. Even when a comparison looks good and logical, we need to examine it more carefully. And we need to examine alternatives more carefully too. We need to keep an open mind, and an open forum for the flow of ideas.
I have no illusions that I see all the great "truths" about the paleo world. I think I see a few of them, but I'm certain I miss more than I see. I am reminded of an intriguing element of Robert Heinlein's classic, "Stranger in a Strange Land" novel. He has a character who is a "Public Witness", a person trained, certified to observe things and give legal testimony to what the Witness sees or perceives. As recounted in the book, if you asked the Public Witness what color a house was, the witness would not testify that "the house is white". Instead, the witness would testify that "The side of the house I saw was white". The witness was trained to understand what he/she did in fact see and know, and equally trained to understand what he/she did not see or know. Like this trained witness, I will tell you I see one side of this house. Standing a one point, I can see only one side, or two at best. I can't see every side from one fixed point. That's why I invite your views. I'd like to see the entire house, and I can't do so without your help.
In closing, the lesson of the Brachiosaurus is that even when you have an attractive and seemingly logical comparison with nature today, try to think it through on a deeper or alternate level. Maybe you'll re-affirm that your original view was correct. And maybe you'll find a deeper truth that helps you understand both past and present better. After all, how many of you reading this knew about nursery herds? I didn't, until I looked deeper into the question of the color of a brachiosaurus.