(Taking from this earlier answer)

From ancient descriptions we have at disposal, ancient southern Britons used body-paint in ceremonial contexts, a practice otherwise unnatested for in the mainland (either in litterature or art representations) except for a brief mention by Tacitus on an otherwise unknown Germanic people with "black shield and painted bodies" in warfare (Tacitus, Germania, XLIII).

Unfortunately, these mentions are rather vague or even seemingly self-contradicting.

The first, and most famous, of these is Caesar's description of his battles against Britons as he led a raid in the island in 54BCE (De Bello Gallico, V, 14)

All the Britons, indeed, dye themselves with vitro which occasions a sky-like (*caeruleum) color, and thereby have a more terrible appearance in fight

Vitrum means 'glass-like' there, the substance used by these warriors being comparable to the hue roman Glass or the sky would have. It's ambiguous whether Caesar meant a light greyish or greenish blue, or a darker share of blue; and besides being informed it was characteristic of these "maritime" peoples (i.e. from south-east Britain, rather than "All the Britons" in the broader sense) we simply don't know what it looked like.

The second important mention of British body-painting comes from Pliny (Natural History; XXII, 2).

In Gaul there is a plant like the plantain, called glastum; with it the wives of the Britons, and their daughters-in-law, stain all the body and at certain religious ceremonies march along naked, with a color resembling that of Ethiopians.

While the account focuses on British women using body-painting to denote their social and marital statuses, the description is a priori similar to what the men bore in warfare (itself a social ritual on its own). Pliny as well informs us that the obtained color was dark enough to be compared to the darker skin shade Sub-Saharian African people had.

Still, which color exactly? And for that matter, what glastum actually was?

It is traditionally identified as woad, and there's a linguistic argument to support this : words in dialectal Gallo-Romance and Retho-Romance to describe blueish (blue-green, blue-grey, dark blue) colors or objects could be related to a reconstructed *glasson in Gaulish with possible cognates in Insular Celtic ( p. Irish glas or Brittonic glas) that share similar, if diverging, meanings in describing sort of blueish color : glastum could thus be a translitteration in Latin of this proposed Gaulish word.That woad cultivation is unnatested in the British Isles is no problem if we consider it was imported from Gaul, whose trade relations with southern Britain were quite important at the turn of the millenium : woad cultivation itself is unnattested in Gaul but remain credible up to a point in the southern regions where cultivation of Mediterranean plants introduced by Greek and Roman trade took plance.

However, Pliny knew what woad is and mentions it as both isatis and vitrum elsewhere (Natural History; VII, 14) : maybe he was simply unaware that the "blue plant" he was told about was woad, but it's not impossible glastum was a different variety (indigo or paor?) or a different plant that caused a blueish color.

Altough we could argue that Caesar's vitrum and Pliny's glastum were two different plants, one of them woad, it might be needlessly complicated and looking too much in vocabulary disrepencies. Let's settle for that southern Britons were said to use a blueish body-paint for ceremonial purposes that could be produced from imported woad, the colour disrepency being maybe explained by different tinctures or mixes.

Some practical motivations were advanced to explain this practices but are generally unconvincing at best.Pliny's description seems to argue for Britons colouring all their bodies with it, without apparent contradiction in Caesar vague's mention, rather than the "ethnic" design with squiggly lines and spirals usually found in modern representations, meaning that tribal/political identification or reclamation would be unlikely (altough quite possibly entranched and used as a regional practice).A common enough argument for glastum/vitrum as woad lies in the antiseptic properties of the plant ancient Britons would have benefited but that seems unlikely as well : not only woad dye is difficult to hold on (rain water is more than enough to clean it off) but it is a caustic product that would have been painful enough and prevented a good cicatrization of wounds.

Eventually, the precise meaning of the custom is lost to us, even if it probably says something about social status and identity of "achieved" man or woman in southern British society.