when we watch them so much we don't notice them fading gradually as they come closer to a shed and as they are already wonderful looking they look as if they were just hatched after a shed. I have been thinking the same thing but I beleive its only natural for any snake to fade with age. Most do, not all. My pastels have kept their brightness for the most partbut they are only 700g and 800g. I'm sure as they grow they will fade a bit also. Maybe you found a line that won't, who knows. Good luck!
Suggestion: Start to take pictures of your pastel in the same light, surroundings with the same camera every week or two. Mark the date and do it for a couple of months. I do it with my two pastels and a weird girl of mine and have a log on my computer. Its kind of cool to see the changes.