The time it takes for them to get their color can vary. If they are getting a lot of live foods like brine shrimp they can grow quickly and get their color at a younger age. The size of the tank can have also have an effect on growth, the larger the tank the better the growth rate. In general, guppies start getting their color in a month or two but don't fully get their colors until they are full grown which can take 3 to 5 months.