Proposed Canon
War Paint is a special material applied to the skin to produce a positive effect.
It is likely magical, though probably a low level. Similar to a Magic Item, changed like raiment. It may be temporary and need re-application on a turn by turn basis. Its effects may include combat bonuses, or adding special abilities.
 Real World References
Wearing paint on the face, body, (and steed), was a common enough practice in many civilizations, and indeed endures today in its fashion (camo paint, eye black on athletes). The belief that the paint would grant magical properties may have been wrong, but the added confidence added to the fear and intimidation of the enemy could have been a real benefit.
At some point "war paint" became a glib and/or derisive colloquialism for cosmetics (particularly women's). The use of cosmetic makeup for "war paint" is a cute reversal.