Organic light emitting devices (OLEDs) are promising candidates for light-weight color flat panel displays. Different device structures with emission in the blue, green, and red spectral region are discussed with respect to their optical and electrical characteristics. Blue OLEDs based on OXD-8 as emitter molecule show quantum efficiencies of 0.9% (2.2 cd/A, 0.6 lm/W), green emitting devices based on Alq3 achieve values of 1.4% (4.9 cd/A, 1.3 lm/W). Electroluminescence with colors tunable from yellow-green to red is obtained with DCM doped Alq3 layers. To investigate the device physics, a thin DCM:Alq3 sensor film is inserted into an Alq3 emitter layer. Position and current dependent spectral characteristics allow to explain the device behavior. Carrier injection, transport, recombination, exciton diffusion and decay are identified as the crucial processes responsible for the operation of OLEDs.