Short Answers:
My fry started getting adult colourings when they were 1.5-2 inches long although they had some colour before that, the males had the neon tints around some of the fins before they were an inch long iirc.
With regards to adults, it can depend on certain things. When I kept them with a white sandy substrate the colours were much paler than when I introduced a dark gravely substrate. Furthermore, when they are sexually aroused and courting their colours become a lot more vivid, more so in the males in my experience but the females' bellies will also get slightly more crimson than usual. Also food may effect the colouring of fish but you should be good with kribs in that regard.
Does that answer your question? and good luck with the Kribs.
Edit: Spelling