Not white and black for franco pls - I like the idea of relatively similar colours for all of them, its what JFD does with his alternate leaders and its cool
JFD has a lot of alternate leaders with different schemes, see: Hannibal, Henry's England, Alexander's Russia, and I'm sure that Jan doesn't choose colour schemes with the intention of looking similar to the original civs in mind, but rather because they fit the civ and look good. Besides, these aren't alternate leaders, they're full fledgling different civs that people will probably want to play with in the same match, so they should have pretty different colour schemes.
Castile could get a red background with yellow symbol like in their coat of arms, but personally I'd make it inverted (Red castle with yellow background like in JFD's Castile). However, this would probably conflict with Aragon and their also yellow-red coat of arms, so it only depends on which one of the two you wish to bestow the coveted yellow-red combo. I associate yellow with Castile more, so I'd give that to them. Paradox made me associate Aragon with brown/orange (Which is the combination of yellow and red, so hey, it fits too
), so I'd probably go with that too, and give Navarra a red background with a different shade of yellow.
Franco would get something low saturated and dead-looking like the other WW2 civs.