Seems to be normal. ^ That is mine which is almost identical to yours and he or she changes colors whenever threatened, eating, or when protecting itself. Seems to be normal and mine is at about 4.5-5. 5" and went through a "grey" stage where it was completely Grey for about a month and seems to be all black for the most part now.
Mine "outgrew" its beautiful colors but has some amazing colors still. Here is a picture of when I got him
He was at about 1.5" and he still switches back sometimes to those colors its pretty awesome! The white one in the pic also has awesome colors
He's about 6" maybe 6.5" they always stick together