IMHO:
rep. spain: dark red color, like burgundy or somewhere near the soviet dark red. as the republicans received plenty of support in men and materiel from the USSR.
nat. spain: somewhere on the grey scale. they got plenty of support from germany, grey is underused, nat. spain is unlikely to become hostile to germany and italy, and after WWII, francoist spain was for all intents and purposes the last fascist regime.
post-SCW spain: yellow or bronze. the same color palette as HOI 3 spains. either one works.
democratic spain (in the event of democrat vs communist follow-up civil war): light or dark blue, whichever is more photogenic and farther away from the french blue.
communist spain (in the event of democrat vs communist follow-up civil war): same as rep spain.