What do the 650 nm, 550 nm, and 450 nm channels measure in the Go Direct Light and Color sensor?

The values 650 nm, 550 nm, and 450 nm refer to the peak wavelengths of visible light to which the color sensors respond. These wavelengths correspond approximately to red, green, and blue.

TIL 3966: Go Direct Light and Color Sensor Troubleshooting and FAQs