One of the reasons we think of clothes worn at that time as drab and dull is that we often see cloth or clothing scraps recovered from archaeological projects and they are dark, brown and dull. This is more the effect of the passing of time than the original state of the garment. Colour was most certainly used and was important. It is certainly true that many people would have worn clothes made from simple woven linen and wool in unbleached, largely undyed colours. While there were a great many vegetable sources that could be used to extract colour - yellow is particularly easy to achieve - many produced a fugitive, weak colour, or inconsistent results. However by the Middle Ages dyeing was a well established practice and there were some excellent strong colours available. All the dyestuff we know was available to the regions you specify are known to produce often excellent but nonetheless varying results. Better dyeing techniques with mordants and metallic salts would have made better, longer lasting colours. Wool dyed more easily than linen, by far.

Colour was important as a signifier of social standing, money and effort were spent trying to get the best colours with the best durability and coverage - the richer you were the better your clothes were dyed - it was not just the rich that used dyes, they just seem to have used the *better* dye techniques. Viking settlements have been extensively shown to have had a thriving trade in importing dyestuffs and mordanting materials, for example a non native variety of clubmoss was discovered at digs in York; it is believed it's high aluminium content made it a useful ingredient for mordanting.

Chemical analysis of garments found in your queried period show that wool was commonly coloured with bedstraw, weld, broom, heather, woad, walnut, madder, various lichens, and kermes. This selection gives you a nice selection of yellow, reds (from orange to red), purple, pink, a good strong blue, brown and black, and green by overdyeing yellow with woad. The use of these is further supported by botanical remains identified by archeobotanists at settlement sites. I've previously answered a question about colours available in the Medieval period generally, https://www.reddit.com/r/AskHistorians/comments/9me5xa/what_colours_of_clothing_were_available_in_the/e7eyuex/ which might give you more colour specific detail, but that reply also includes dyestuffs that were definitely not available in your chosen period.

Availability of dyestuff and associated dyeing materials probably hugely influences popular colours in any given area. Penelope Walton's "Dyes in the Viking Age: A summary of Recent Work*"* in the published papers presented at Dyes in History and Archaeology 7 is great if you want a really detailed spectrochemical analysis from actual surviving remnants. Walton has many papers on the subject and generally has observed a preference for red in Danish finds , blue/purples in Irish, and blue/greens in Norway/Sweden.

Some other useful sources:
A.R. Hall Dyes on Historical and Archaeological Textiles "Evidence of Dyeplants from Viking Age York and Medieval Beverley"
Frances Pritchard "Late Saxon Textiles from the City of London." Medieval Archaeology 28 and "Aspects of the Wool Textiles from Viking Age Dublin." Archaeological Textiles in Northern Europe: Report from the 4th NESAT Symposium

Elizabeth Heckett " Some Hiberno-Norse Headcoverings from Fishamble Street and St. John's Lane, Dublin" Textile History 18