Well, according to "De Bello Gallicio" by Julius Caesar, he describe the paint that britons used as blue-dye.... maybe we could refer to Isatis Tinctoria for the source of that dye... but from their purpose, and the description that they are long lasted when used in battle (natural dye (organic) from plants are suspectible to be dissloved in sweats, because the formic acid present....), gives the probability that they are also mixed with copper or iron based mineral dye.... (note, the reason I said about the latter IS hypothesis, is because if it was used on purpose of creating a magical pattern that protect the user... it isn't funny when you are sweating and your dyes are gone...)

From Wikipedia:
Woad is native to the steppe and desert zones of the Caucasus, Central Asia to eastern Siberia and Western Asia (Hegi), but is now found in southeastern and some parts of Central Europe as well. It has been cultivated throughout Europe, especially in Western and southern Europe, since ancient times.
They are long used for dyeing textiles and some products... even found as painting, maybe their most famous usage was for warpaint... but with their extremely wide usage on britons, one can bet safely that they are also used as dye for cloth and trousers. As making a paint that only usable in war / battle is non - economic for such large scale.

And while I didn't know much about their religious use, it was a logical assumption that they was used not only in war... but also in peace to create some fancy blue trousers....