Yeah, it unifies them a bit, I think...
That new color is darker than Xivilai's hair, though, so it doesn't work as well there. You may want to integrate the two better somehow, like, using some of that dark red in the hair. But... Maybe it's a bit too saturated to be in the shadow.
As for choosing and checking colors, I have no idea, so I can't help you with that, unfortunately. I use other people's palettes most of the time, but I don't do much pixel art anyways.
Anatomy-wise it looks much better on the back, but it kinda breaks when you get to the right arm and hand.
Watch this video from Proko to learn more about the form of the lower arm and its muscles:
https://www.youtube.com/watch?v=pPWmJj9l1_0And whenever you can, watch these playlists to learn more about how to draw the human figure in general:
https://www.youtube.com/playlist?list=PLtG4P3lq8RHGuMuprDarMz_Y9Fbw_d2wshttps://www.youtube.com/playlist?list=PLtG4P3lq8RHFBeVaruf2JjyQmZJH4__Zv---
I think that some forms work better as a unified mass rather than as individual muscles. For instance, you don't have to draw the separation between the hamstring muscles. But if you do, then they should reflect the overall form of the leg.
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There:

I tried making the right arm look more natural, even if not as relaxed as in my first revision (which I prefer).
But of course it all depends on what you want him to look like he's doing. And for that you're better off basing the pose on a photo rather than on a 3D model (if that's what you are doing).
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I've been thinking about that ear too... You could make it so that they wouldn't be as parallel to the side of the head, then the head wouldn't seem to be so much in profile, rather in a 3/4 view from the back.
And you could make the other ear peek a bit from the other side.
https://i.imgur.com/oNBJFve.png https://i.imgur.com/cY7usT7.gifEDIT: Actually, that right arm looks way too bulbous, and either too close to the body, or the elbow is too far back.
I "fixed" it, while at the same time making him not as bulky as before:

Changed the left arm as well to a more symmetric pose.
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EDIT 2: Except now the legs look way too thick for the rest of the body.

There:
