On the History Channel's series for TV, "The Vikings" the characters are dressed mostly in drab and grey colors. There were other colors available, and I don't know why they chose to portray the costumes so dark. Here are some sources from the WWW illuminating the bright colors obtainable from the native plants.
Fibers Help Update the Rise of Culture, By Henry A. Shull Harvard Crimson September 25, 2009
http://www.thecrimson.com/article/2009/9/25/fibers-help-date-rise-of-culture/
Viking Age Dyestuffs
http://www.cs.vassar.edu/~capriest/vikdyes.html
http://www.hurstwic.org/history/articles/daily_living/text/clothing.htm
http://www.42nd-dimension.com/NFPS/nfps_colours.html
Interesting Q & A on Viking Textiles with nice Color Wheels of wool and information on plants and methods
http://www.bushcraftuk.com/forum/showthread.php?t=54737
