Blue was also a tough one. Most common blue dye was indigo/woad (same pigment produced by two different plants), which due to its nature needed to be applied in a different manner from other dyes. Ultramarine blue existed for painting, but it was expensive (pigment made from crushed semiprecious stone). It wasn’t until the Industrial Revolution when chemists started working with coal tar (byproduct of the local gasworks) that a cheap, stable blue dye (Prussian blue) became available.