The Scottish used the Woad Plant, while the Roman Julius Caesar thought the Picts used some type of metal. Here is some interesting info from Wikipedia.
Woad (or glastum) is the common name of the flowering plant Isatis tinctoria in the family Brassicaceae. It is commonly called dyer's woad, and sometimes incorrectly listed as Isatis indigotica (a newer and invalid name for the same plant). It is occasionally known as Asp of Jerusalem. Woad is also the name of a blue dye produced from the plant. Woad is pronounced /ˈwoʊd/, to rhyme with road.
In Europe, woad was the only source for blue dye available until the end of the sixteenth century when trade routes began bringing indigo from the far east.[citation needed]
The first archaeological finds of woad seeds date to the Neolithic and have been found in the French cave of l'Audoste, Bouches du Rhone (France). Named Färberwaid (Isatis tinctoria L.) or German Indigo of the plant family (Brassicaceae), in the Iron Age settlement of the Heuneburg, Germany, impressions of the seeds have been found on pottery.
Julius Caesar tells us (in De Bello Gallico) that the Britanni used to colour their bodies blue with vitrum, a word that roughly translates to "glass". While many have assumed vitrum or vitro refers to woad, and this misconception was probably repeated for political reasons,[1] it is probable that Caesar was describing some form of copper- or iron-based pigment.[1] The northern inhabitants of Britain came to be known as Picts (Picti), which means "painted ones" in Latin, due to these accounts of them painting or tattooing their bodies.
In Viking age levels at York, a dye shop with remains of both woad and madder dating from the tenth century have been excavated.
Woad and health
Recently, scientists have discovered woad might be used to prevent cancer, having more than 20 times the amount of glucobrassicin contained in broccoli.[7] Young leaves when damaged can produce more glucobrassicin, up to 65 times as much.[8]
Indigowoad Root (Chinese: 板藍根; pinyin: bǎn lán gēn) is a traditional Chinese medicine herb that comes from the roots of woad, but often incorrectly listed under the synonymic name, Isatis indigotica. It is also known as Radix isatidis. The herb is cultivated in various regions of northern China, namely Hebei, Beijing, Heilongjiang, Henan, Jiangsu, and Gansu. The roots are harvested during the autumn and dried. The dried root is then processed into granules, which are most commonly consumed dissolved in hot water or tea. The product, called Banlangen Keli, is very popular throughout China, and used to remove toxic heat, soothe sore throat and to treat influenza, measles, mumps, syphilis, or scarlet fever. It is also used for pharyngitis, laryngitis, erysipelas, and carbuncle, and to prevent hepatitis A, epidemic meningitis, cancer and inflammation. Possible minor side effects include allergic reactions and dizziness; only large dosages or long term usage can be toxic to the kidneys. These treatments have not generally been evaluated clinically.