...Are you sure what you mean isn't, "The colours I'm expecting to see used would look a lot more, well, faded by hundreds of years of weather"?
'Cause they had plenty of vibrant colours in the medieval period (not as vibrant as our neon stuff today, sure, but plenty bright). It's just that a) it was so expensive only nobles (y'know, the people we play
) could use them, and b) most authentic examples you see today of objects dyed during that period are horribly faded (because it's been nearly a millennium).
Doesn't mean they didn't have bright, sharp colours.