ANSWER: Hello!
It is different with all platies but generally speaking, the gravid spot goes through 4 color changes.
First, it will be a dark black/dark brown, then after about 3 weeks it will turn red/pink, then it will turn red/black right a few days before the babies are born, the after the babies are born it will turn into a very light pink color before the cycle starts all over again.
-Nick
---------- FOLLOW-UP ----------
QUESTION: Hello again. How long until the red pink spot turns red black?
Answer
Like I said, that matters a lot on the individual fish, and there are no real set guidelines.
The general gestation period is about 1 month, and the fish is probably not pregnant for 3-10 days after it has it's babies.
Let me know if that helps, if not I'll try to clarify or find a more specific date.