This is coming to you late but I'm sorry you didn't get a reply when you originally posted.

When you say stage 50 I assume you're speaking of Gosner stages? Most people will not know what those are. Development speed and when certain changes take place are highly dependent on temperature, and to some degree, food.

Color doesn't necessarily mean a lot in tadpoles. If you could post a photo we could give a better assessment.

Again, sorry for the delay in response. Every now and then a post slips through the cracks.