When do baby guppies get their colour?

It depends on the strain of guppy you have, I've seen some males begin to get their color after 4 or 5 weeks, others 2 - 3 months. Keep in mind, much depends on how fast they grow, if you are serious about raising guppies and want them to reach full size, sooner and be very healthy, you need to do a few things. Water temp should be around 80 degrees, weekly water changes, fry should be fed a little bit every few hours, BBB (baby brine shrimp) or microworms at least twice a day, powdered flake food other times.