You can put a S1 (early S2) ECU in the later RS's but not the other way round as there is no knock sensor and the ECU needs one plugged in or the timing will be hugely retarded!
Blue-Red-Blue = S1's (B, C, D)
Blue-Red-White = Early S2's, with no knock sensor, mainly the first D plates
Blue-Red-Black = S2's with a knock sensor (E, F, G)
Hearts & Diamonds = 90 spec, with a knock sensor (G, H)