Depends on the car. My personal favorites for the Triumph Line are as follows (I'm NOT gonna list them all!), and they seems to match the top selling prices.
Spitfire - White or Red, yellow is hit/miss. Racing Stripes, done right, does not devalue, like White with 2 red racing stripes, or red with 2 white. If stripes are done, do not off-set or do a single off-center stripe, as it makes it much harder to sell.
TR6 - Light Blue (French Racing Blue?), British Racing Green, Yellow
TR4 - Light Blue (FRB) or BRG
TR3 - Off White (egg shell), Red, FRB, BRG, Black. Black with red interior seems to sell for highest, followed close by off white w/ tan. Very classy combos!
TR2 - Same as TR3, but factory shades only.
TR7/8 - Bright Colors. Dark ones sell for peanuts. White is hit/miss