Well, that's the other thing, of course.
We tend to paint our medievals in bright colours but the truth is they wouldn't have been - they didn't many effective mordants to fix dyes so clothes would have washed out very quickly.
That drab look is what everyone would have been wearing. It's just that the nobles would have been wearing clothes with washed out expensive colours (reds and blues) and the peasants would have been wearing washed out cheap colours (earth tones). But still washed out.
Again, they would have all looked the same.
But, as wargamers, we want a 'look' to our toy soldiers. So we paint them brightly, or we give give celtic nations swirly designs on their shield to make them different, or put them in some form of plaid.
And that's perfectly valid - we all do the same.