AI made a cake this weekend using a bright green buttercream (crusting, if it matters). Parts of the cake are darker then others (like in swirls or something). I didn't use two different colours, just the one shade. Maybe is it because I put it in the fridge to set up, then fixed up some areas with more buttercream and the bench scraper. Does this happen to anyone else? Is there any way to fix it or will it fix on its own when out of the fridge for a while?

AWhen the buttercream dries out it will change color a little. If the cake was in the fridge the surface will dry and the color won't be the same as the icing that the icing you've set aside for later use. I don't know if leaving it will make the color all fade to the same tone, I would think that would depend on the color. Not really a lot you can do about it..