Colour emphasis on the NES is accomplished via bits 5, 6, and 7 of PPUMASK, for red, green, and blue respectively. These bits enable/disable an "emphasis" or "tint" effect visually on screen.
On Dendy and PAL PPUs, the purpose of bits 5 and 6 are reversed (red for green, and green for red).[1][2]
See NTSC video for a description of how bits 5-7 work on NTSC and PAL PPUs.
The RGB PPU used by PlayChoice and some other systems treat the emphasis bits differently. Instead of darkening other RGB components, it forces one component to maximum brightness. A few games, which set all three tint bits to darken all colors, are unplayable on these PPUs because they instead turn the entire screen white.