Well, the short answer is 'we don't really know, but probably a wide variety of naturally occurring colours and, for fancy stuff, man-made dyes.' The longer answer is that we know that in early medieval Scandinavia the use of dyes was fairly prevalent. In the sagas, we have mention of characters wearing brightly coloured clothes, like Ólafr pái (Olaf the Peacock) and others dressed all in black, like Egill Skallagrímsson (whose black cloak was lined in scarlet, just to remind you of how wealthy he was.)

Scarlet, in particular, seems to be a popular choice for wealthy Icelanders to wear (Egill, Gunnlaugr, etc.) as are embroidered tunics.

If you're interested in the symbolism of colour in Old Norse, Kirsten Wolf has done a lot of work on it, and you wouldn't be remiss in reading some of her articles.