Really fun game! I used VGVubmVycw== to solve it, but I think I misunderstood how Yellow worked. The rule I came to the conclusion was dGhhdCBpdCBoYXMgdG8gaW5jbHVkZSAiRVJTIg==, but if a word like cG9wY29ybg== works, that rule completely goes out the window.
(Every instance of jumbled text is Base64.)
Edit: Oh wow, I think I know how Yellow works now, and I misunderstood it more than I originally thought. Huh.