well...... each of the authors wrote the different books, since Erin Hunter is a pen name for a group of people. They used blue, green, and i think another color. Anyways, they should be green, according to the wiki which also has a cited source for that.
As for scourge's collar, i don't really know if that was ever mentioned or not, but many people think it was purple or something, and I kinda like that idead too.
It's been a long time since I read Warriors, and now it can be a little confusing!