From having spent far, far too many hours hunched over little metal miniature 18th century figures painting them, I can tell you what combinations make distinctive and even attractive combinations, because they were all used by one unit or many in Europe: For a primary color of Red, for instance
Red - Dark Blue
Red - Light Blue
Red - Dark Brown
Red - Light Green
Red - Yellow
Red - Black - BUT black and white seem to be reseerved for Minor Powers in Civ VII, so it's probably a no-go in the game.
Other primary (uniform) colors were:
Dark Blue, Light Blue, Gray, White (BUT see above, probably only for Minors) and medium Green
Contrasting ('distinctives') colors most used were:
Dark Blue, Light Blue, Red, Yellow, Brown, Light Green, Dark Green
Less often used, because the colors were so 'fugitive':
Orange, Lavender, Pink, and the orangish-brown color called by the French 'Feuille Morte' (Dead Leaf), which they used on uniforms and flags when they ran out of combinations of primary colors.
Even without subtler distinctions of shading like mauve, maroon, magenta and such, that still gives us enough combinations to distinguish at least the Civs/Leaders on release.