Depends, with a good diet 8-10 weeks, some may take longer.. i have had guppies not show little color after 10 weeks and all of a sudden 1 week later, they are amazingly all bright with loads of color..
Usually though, a good diet and decent size tank and they will show after 8 weeks..
Depends on color. Yellow's start to show at 4-5 week's. Reds start to show at 6-7+ week's, oranges and pink's start at 4-5 week's. Im not sure about blues. though.