emperor angel
 

how big are emperors usually before they start transitioning into their adult colors. is there a way to encourage the change?

From what I have seen and read, they usually start to turn into adult color and pattern at about 3"-4". Be sure to feed variety, high qulity foods if you have a juv. color emperor angel. This will not only encourage it to grow and also increase the chance that it turns into normal adult color and pattern. I have read a lot of sad stories about juv. emperor angels not being able to turn into bright, normal adult color and pattern like a wild caught adult emperor. I assume there are something missing in an enclosed environment. Either diet, chemical or environment related.
Good luck.