Had another go at it with Resolve:
Tried turning the turtle shell colors a bit browner, but then the rust tint starts to come through more in the shadow from the rocks:
So there is a trade-off.
Maybe needs brightening a tad, but in terms of color balance, better results all around. What's more, all done with the Channel Mixer - no secondary tweaking with Color Temp/Tint, the Color Wheels or qualifiers. After tuning the sub-channel biases some more just needed finer balancing of the primary R,G,B channels. Then levels, contrast and mid-tone detail and partial desaturation of the shadows. Also used the denoised (SMDegrain) clip as source here.
With that realization, I'm more confident about achieving a similar result with the Channel Mixer in AVISynth.