"Colour+Brightnes"s means "normal" motion detection. This is compared to "Color only" and "Brightness only" modes.
"Color only" mode makes
Blue Iris ignore changes in brightness that do not also change the color. For example on a cloudy day the brightness outside might change a lot and normally this would cause motion detection to trigger when you don't want it to. But in "Color only" mode, the motion sensor less likely to trigger because the color stays mostly the same as clouds move in front of the sun.
"Brightness only" mode is far less useful. It means that Blue Iris treats each camera frame as black and white (and gray) for the purposes of motion detection. For most real-world situations, the effect is about the same as using "Color+Brightness" mode. Nonetheless I use "Brightness only" mode in my Night profile on the offchance that it will work better than "Color+Brightness" mode. All my cameras switch to black and white mode at night anyway.