The RGBI signal is CGA compatible and its four bit output represents 16 colors (2^4).
Denise outputs 4 bits each for R, G and B that are input to the Vidiot DAC device and then output as analogue signals. The most signficant bits of each color are also output as DR, DG, DB plus the least significant bit from B as DI (intensity), doubling or halving the brightness of the output pixel. The colors are quite ugly, so the signal's not of much use:
