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.
- Bit 5 emphasizes red on the NTSC PPU, and green on the PAL & Dendy PPUs.
- Bit 6 emphasizes green on the NTSC PPU, and red on the PAL & Dendy PPUs.
- Bit 7 emphasizes blue on the NTSC, PAL, & Dendy PPUs.
- Each bit emphasizes 1 color while darkening the other two. Setting all three emphasis bits will darken colors $00-$0D, $10-$1D, $20-$2D, and $30-$3D (see PPU palettes). Note that $1D black is affected by color emphasis, but $0F black is not.
- The emphasis bits are applied independently of greyscale (bit 0 of PPUMASK), so they will still tint the color of the grey image.
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.
- NTSC video
- PPU palettes
- PPU registers
- Colour-emphasis games
- Color $0D games
- PAL PPU swaps green and red emphasis bits: http://forums.nesdev.com/viewtopic.php?p=131889#p13188
- Dendy PPU swaps green and red emphasis bits: http://forums.nesdev.com/viewtopic.php?p=155513#p155513