My platy fry all have some color at birth, but it gets more intense over time. They should be full color, or close to it, by two months. The first platy fry I noticed in my community tank was a yellow-gold color when I first saw him, with no mickey mouse spot, although his mom is an orange/red mickey mouse. He is now about 6 1/2 months old, and he is more intensely colored than the females, and does have the MM markings. I thought at first that he was the only fry born, because I never saw any others. I have since discovered that he was way too big for a newborn platy - he was probably at least 10 days old, maybe as old as 2 1/2 weeks when I noticed him, and I assume he had siblings who didn't make it in the tank. A few days after I first saw him, I picked up 3 neon black painted platy fry (yellow/gold to pale orange where they didn't have the neon black scales) a bit smaller than he was, and another that was at the time yellow. He is now bright orange/red, and the 3 neon black painteds are orange to red where they aren't neon black.
I have about 75-80 platy fry at the moment, from two drops. The older ones are 37 days old, and are mostly yellow, which they were at birth. Some are beginning to show darker shades of gold and orange. Their mother is a sunset platy, and their father is either the bright orange platy or the orange/red mickey mouse, probably the mickey mouse. Therefore, they could end up anywhere from the yellow blending to orange (front to back) of their mother, or a darker, brighter orange, with or without the MM markings. The younger fry are about 3 1/2 weeks old, and most of them have the neon black scales, with various shades from yellow to dark orange on the rest of their bodies. The black scales were evident at birth, and the areas that weren't black were whitish to yellow. Some are already quite orange. Their mother was an orange/red mickey mouse, so obviously their father was the male neon black painted platy.