Dimming Curves on LEDs

The real issue is the lack of calibration in the cheap LED fixtures, so the colour produced with 50% red and 50% blue is not the same as 100% red and 100% blue, because the blue LED at half power is producing less light than the red at half power, so the colour is more red.

More expensive fixtures have calibration curves loaded into software in the lamp that fixes that and for example drives the blue LED with 60% power when sent 50% request on DMX. The really good fixtures are individually calibrated at the factory with those curves stored in flash memory on the fixture as different batches of the actual LED bulbs (COBs) will have slightly different characteristics.

There could be advantages of there being a separate intensitry channel as at low levels there is a lack of precision if the value for RGB is computed at the desk rather than at the light. eg suppose you were at 100% red and 50% blue and you dimmed that down to say 5% the desk would have to send DMX values of 255 x 5% = 13 for red and 255 x 50% * 5% = 6 for blue so now the red has ended up 8% higher than it should have. If the calculation was done in the fixture then it wouldn't be limited to just the 255 values for the red and blue (assuming the fixture maker could be bothered to do the conversion properly). Obviously if the fixture has 16 bit RGB this does exist as a problem but you only get that on the good fixtures and they'll have intensity anyway.