I don’t know anything about Welsh sumptuary laws so cannot comment on social restrictions on clothing colours, but this link lists some of the dyes available so you can work out the range of colours available: http://www.cpgt2005.madasafish.com/PDF/Dye%20plants.pdf
It’s worth remembering that using different mordants can affect the final colour of the dye as well as its colourfastness. Generally speaking, I would say that a full range of colours could have been used, based on the available dyes. That means you should be fairly free to use whatever you want, although dayglo colours would be right out.
These colours are what the Vikings could have had using dyes in Scandinavia. Some of those dyes are the same as those available in Wales, and there is no reason why wealthier Welsh people would not have imported dyes or dyed cloth too.
Never argue with an idiot. They'll only drag you down to their level and beat you with experience.
