However, utilizing another's work for educational purposes is a great idea. Some don't think that's the case (well, he's kind of against the selling of copied art, which I agree with), but in art, at least, copying others has long been one of the best ways to learn things yourself. In fact, since 1793
[The Louvre] has allowed, even encouraged, artists to hone their skills by copying the masterpieces in its collections. Thousands have done so, including great classical painters from Turner to Ingres, Impressionists from Manet to Degas, and modernists like Chagall and Giacometti. “You have to copy and recopy the masters,” Degas insisted, “and it’s only after having proved oneself as a good copyist that you can reasonably try to do a still life of a radish.”
On the other hand, when trapped inside the whale, the light is no longer the bright sun shining through clear water. Instead, it is a low yellow, tempered by the blues of the baleen. What's the neatest to me, though, is the light reflection off of the whale's tongue onto Marlin's back fin. Also, the brightness of the outlines of Marlin and Dory, showing off their 'shininess' when they are in the air instead of the water.
The last one I did was mostly for color and composition. The murky green palette does a couple things. First, it's somewhat dank and dreary. No one likes green water, because it reminds you of the scummy, polluted, industrial water of cities and mucky ponds. You don't think of vitality there. Second, it emphasizes how far the tropical fish have come. Dory's bright blue and Nemo's bright orange stand in stark contrast to the green. They don't blend or match their surroundings. They are in a foreign place. The crab, on the other hand, is also a muted yellowy color, which shows that he's right at home where he is. Third, it diffuses the light in a neat way. Because they are swimming above/standing on a metal pipe, the light is reflecting slightly off of it, but only if you are close enough. That's why Dory gets a little bit of that reflection and the crab gets a lot. Nemo, however, doesn't get any. He's too far away for that reflected light to reach him.
So, go and copy! Copy, copy, and copy! Maybe you'll learn something, and then you'll create your own stuff that other people will copy, and copy, and copy!