One of the most commonly and frequently used colors in Viking Age textiles was blue, derived from a plant called woad. While wool fabrics absorbed most colors readily, that was not the case for linen, which could only dependably dye blue, and sometimes red (madder). The Vikings were not the only civilization to utilize this blue in their articles of clothing, as we will explore below.
Woad, the plant that the Vikings used to produce blue were found as far back as the Neolithic, having been found on shards of pottery in the Iron Age settlement of Heuneburg (now Germany). Analysis of cloth wrappings have also concluded that the Ancient Egyptians used woad to color their fabric. 
Woad (Isatis tinctoria) is one of the many plants that produce Indigotin, the chemical compound that creates the color blue. However, woad is not the plant with the highest concentration of the dye compound; indigo from eastern parts of the world actually have a higher concentration of indigotin than the woad that was traditionally grown in Europe.
In 1498, when Portuguese explorer Vasco da Gama opened up direct trade with India, woad growers and merchants in France, Germany, and England worried about how this new indigo plant would negatively impact their business. This was because the new dye from the East was superior, providing deeper and more colorfast dyes than their European counterparts. So began a propaganda campaign which purported that indigo was a dangerous product, the devil’s food, drug, color, and dye, and that it would rot the wool. This led to prohibitions against the import and usage of indigo, enforced by steep fines and even imprisonment and death…
“Whereas of late years there hath been brought into this Realm of England, from beyond the seas, [indigo]…and the colors made from the said stuff is false and deceitful, and are not onely sold and uttered to the great deceit of the Queens loving subjects, within this realm of England, but also beyond the Seas, to the great discredit and slander…of the Merchants, as the Dyers of this Realm…be it ordained, enacted, and established, that all such [indigo] in whose hands soever shall be found…shall be forfeited, and openly burned by the authority of the Mayor…and upon pain that the Dyer of every thing so dyed, shall forfeit the value of the thing so dyed…and the party offending…to remain in prison without bail…till he have satisfied the same value.” – Prohibition against the use of indigo and other exotic dyes in England by Queen Elizabeth I, 1581 
Even with prohibitions and bans in place, the flow of indigo trade could not be stemmed; by the late 17th Century, indigo had become the preferred dye for the color blue, which was evident with the economic impact; the German woad industry collapsed, and a large population who depended on woad for their livelihood, was plunged into poverty. 
Since woad was the predominant source for indigotin and blue dyes in Europe for my relevant time period, I will be doing some experiments with woad dye vats. I will be consulting a research paper, published in the Journal of Archaeological Science: Reports, titled “Searching for blue: Experiments with woad fermentation vats and an explanation of the colours through dye analysis”. This will be part of an A&S submission, where I will spin period-accurate sheep fiber using a drop spindle, dye them using historically appropriate methods, and weave a tablet-woven belt based off of one of the dig site finds. Hopefully the weather will warm up so that I can get a fermentation vat going soon.
Yours in service,
Lady Mathilde Huldsdotter, AoA
 Melo, J. Sérgio Seixas De, Raquel Rondão, Hugh D. Burrows, Maria J. Melo, Suppiah Navaratnam, Ruth Edge, and Gundula Voss. “Spectral and Photophysical Studies of Substituted Indigo Derivatives in Their Keto Forms.” ChemPhysChem 7.11 (2006): 2303-311. Web.
 Lucas, A., and J. R. Harris. Ancient Egyptian Materials and Industries. London: E. Arnold, 1962. Print.
 Hurry, Jamieson B. The Woad Plant and Its Dye. London: Oxford UP, 1930. Print.
 Arsenault, Natalie, and Christopher Rose, Allegra Azulay, and Rachel Meyer. Explorers, Traders & Immigrants: Tracking the Cultural and Social Impacts of the Global Commodity Trade. Austin: Hemispheres, 2007. Print.