This would be better in the support section which is for asking questions. :)
If you're looking to make your pet's color a dye kit color, you have to purchase a dye kit at the black market or by searching dye kit on the search bar at the top and buying them from the shops listed. If you mean a color that can be hatched, you have to save up for a Natural Colored Jelly Bean which turns your pet a random color that can be hatched besides the color it currently is and natural. You can't choose it though, so it may be pricey if you're aiming for a certain color.