Probably because you use more pixels in the second scenario (RRGGBB) to create a “RGB”/“white” pixel. Light is white (and the sun is white too lol). Your white light is created by 3 pixels in the 1st case while it uses 6 in the 2nd.
You also switch off a full pixel, that’s 1/3 of the area needed to create a full white while in the 2nd case it’s 1/6. It’s pretty clear. You switch off a full 2 pixels in the same area the other scenario will approximately switch off 1.
Expand your 1st scenario to the same area the 2nd works and it will explain itself.