I'm planning to assemble a set of armor/apparel of an average 15th c. European man-at-arms. Since most of it would be custom-made anyway, and since this is a long-term hobby project, I want to make sure it's as historically accurate as I can manage.
Ignoring the specifics of the garments for the moment (though I might ask for recommendations here later), I want to make sure I'm sticking with colors that would have been economically feasible and preferably common.
What dyes would have been most available in continental Europe in the Middle Ages? Are there any colors to avoid? I'm personally fond of gray, dark blue, brown, and dark green, but most of what I see in Medieval artwork is pretty bright colored.
The most expensive dyes were purple, crimson, and dark blue. Dark blue was made from indigo, which had to be imported from India; deep scarlets and crimsons made from Kermes; and purple was made by mixing indigo and kermes. As a man-at-arms, you'd probably be of a very minor nobility or gentry, so sumptuary laws would probably prohibit you from wearing these colors, even assuming that you could afford them.
There were, however, cheaper options for red and blue, namely dyer's bugloss, dyer's madder, and woad. Green dye in England was made from mixing woad with dyers' broom. The most common and inexpensive dye was yellow, which could be made from a lot of different plants and flowers.
Unless you're going to be dressing as a serf, you'll want your dye to be as vivid as possible. The more vivid a dye, the more expensive it was, thus even members of the lesser and untitled nobility wouldn't dream of wearing pastels.
Sorry that this is coming so long after the fact, but I recently wrote an answer that might be useful to you:
Why was black so ubiquitous in 16th century attire?
(Ignore the date: I'm mainly talking about changes in the 14th and 15th centuries.)