Our two and a half year old Pastel Yellowbelly has more intense yellows than our Firefly. He's still yellow, but I would almost call it more of a pastel yellow!
I think it depends on what color you're going for. Adult pastel yellow belly tend to be more yellow but darken up a good bit. Firefly stays a lighter color but not as yellow, more of a cream color. If you want a snake that stays true yellow and black as an adult, you won't find anything better than pastel desert ghost, they are also better contrast and much cleaner.