There's a whole lot of pictorial evidence from medieval times and it typically shows quite a bit of fancy colored clothing. Even traditional "folk" attire, that would've been restricted to cheaper choices, is not really all that brownish. Modern chemical dyes were only around starting in the mid-19th c. or so, hence prior to that you got a somewhat poorer color selection, but some variety was absolutely available.

Absolutely, all natural colors would be overwhelmingly available. But even brown is not the natural state of most clothes, so I’m not sure why that is surprising.

Good point, I mostly look at written history but art is a valid source that I forgot about.