If you mean jacksonii xanth. their adult colors are mostly green, of various shades, brown, hints of yellow, hints of blue (rare), hints or orange.
When they are stressed, excited, etc. they change from bright green (resting color) to a sort of patchy dark green and brown.
Baby jax are born a dark color, and they lighten up after a while (don't know how long!).
Baby jax stress VERY easy, which is one reason for their high mortality rate until 3 months of age.
The stress kills them because it effects their immune system.
It's best to keep your baby in a cage with lots of foliage, so he can stay hidden and feel secure. And do not try to handle him.
Maybe someone who breeds them can chime in and give you an idea when his baby colors will change.
All the jax I have owned were juvies of at least 6 months when I got them, so I don't know.