Easiest option is to go for:

Blue-Red-Blue = Series one cars (B, C, D)
Blue-Red-White = Early series two cars, with no knock sensor, mainly the first D plates

or if you have a knock sensor on your car and want to keep its function or can get these ECU's easier (remember they only work with knock senor equipped cars)

Blue-Red-Black = Series 2 cars with a knock sensor (E, F, G)
Hearts & Diamonds = 90 spec cars, with a knock sensor (G, H)