I don't know what the official name for these guys might be, but I will just refer to them as Rhinox :-) These big beasties were fun to paint, and provided plenty of opportunities for subtle transitions in various shades of umbers and burnt sienna.
I think that you can see the greenish gray tones in the shadows, which is something I have been emphasizing in my tutorial videos lately. That is, having more than just a darker version of the same brown in your shadow areas. For instance, if you have a reddish cloak, some deep grayish green in the shadows can provide much more interest and depth.
In this instance, that grayish green helps make the reddish burnt sienna pop much more, since they are opposites. As a result, I get more contrast without having to make the color significantly lighter.
This is part of the triple play of contrast that I mention in my painting videos over and over. Contrast gained by saturated vs unstaturated colors, warm vs cool, and light vs dark.