First off, this is an awesome color scheme, and it is quite well executed!
I only see some slight things that bother my eye, and they are probably just my opinion.
1. I’m not certain of the recesses on his helmet being orange, maybe if you went for a glow effect, but just a flat orange doesn’t quite work.
2. Perhaps try slightly thinner sept markings? They seem slightly bulky and are looking a bit too much like the colored armor panels.
3. The white on the Tau sept shoulder pad symbol just doesn’t look right. It clashes with the color scheme; I'd suggest changing it to orange.
4. Edited in: on the subject of sept markings, (and this is very opinion based) you should always try to have them pointing either upwards, or towards the rear of the model, I don’t know why, but whenever they point sideways, or forwards it just doesn’t look right (although in rare occasions, it does work out).
Also, as far as the two prototypes are concerned, I think I like the original (right) one better, the yellow shading is just plain beautiful. Perhaps try a hybrid, using an orange base, yellow shading, and those whitish highlights?
All said, it's a piece of very nice work, and these are only my opinions.
