Blue, yellow and red were three of the main colours of the Iron Age. The root of the madder plant produced a red colour, woad offered blue and yellow was derived from weld. Plants were soaked in hot water and wool was immersed in the dye bath. Stale urine may have been used as a mordant to fix dyes.
Diodorus Siculus, writing in the first century BC, described the Celts wearing 'shirts which have been dyed and embroidered in various colours'.