HDMI to RGB conversion
The 24 bit data is split into 8 bits red, green and blue. 8b colour means that there are 256 different possible colours.

So if you drop 1 lsb off red and blue and 2 lsb off the green (for example), then you are only going to have 64 possible shades of green instead of 255. 

This article compares the difference between 8b and 6b visually.