The blue face paint, likely "ink" from woad, was worn by several Celtic and Brittanic tribes.
Julius Caesar in his "The Gallic Wars" said, "Omnes vero se Britanni vitro inficiunt, quod caeruleum efficit colorem." which is commonly translated as "all the Brittani, indeed, dye themselves with woad, which produces a dark blue coloring."
There is some debate to this and also debate to the naming of the Pict tribes which in Latin means "painted men". There is no question however, that many Brittanic tribes were tattooed or painted in some way. Julius Caesar also states in describing tribes from around where the Picts were, that they had, "designs carved into their faces by iron." So its possible that woad was not used and the markings on the bodies of the Britanni was actually ritual scarring of some sort.
As for the Roman enemies in Britain, the list is quite long, as nearly every Britannic tribe considered themselves enemies of Rome at some point. So the following is just a rough outline of the most famous that can be expanded upon. ....
55 - 54 BC - Caesar's Invasion - Many tribes resisted Roman invasion but the main unifying force were the Catuvellauni under the command of Cassivelaunus.
43 AD - Claudian Invasion - Again the Catuvellauni were a principal player under the command of Togodumnus and Caratacus.
60 AD - Queen Boudicca of the Iceni led a revolt against Roman occupation and destroyed several Roman colonies and the 9th legion before her defeat.
After the Boudiccan revolt the conquest of Britain essentially became a tribe by tribe Romanization plan. As the bulk of Britain south of Caledonia was subdued, Rome's principal enemies in Brittania were the northern Picts and Celtic tribes such as the Caledones.
Among the most famous leaders of these was Calgacus, who united many of the northern tribes against the invasion by Gnaeus Julius Agricola between 80 and 83 AD. A full account of this, by Tacitus, can be read at
http://www.unrv.com/...tusagricola.php