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I've read that unused bytes of the palettes in VRAM, I.E. $3F04, $3F08, $3F0C and their mirrors, are general purpose 6bit registers.
So, they are just 6bit memory positions that can be read and written through $2006 / $2007 ?
Does anyone recall debugging / disassembling a game that used those registers?

I doubt anybody was THAT short on normal RAM.

Yes. The only way to display those color is to use forced VBlank and point at them with $2006.

Bregalad wrote:Yes. The only way to display those color is to use forced VBlank and point at them with $2006.

Please explain further.
Colors? you mean use them as actual palette indexes?

If you turn off rendering and point the VRAM address at a color, the color will display on the screen, even if it is one of the ordinarily unused alternate background colors (CGRAM $3F04, $3F08, $3F0C). This is the only way to put those three CGRAM entries through the video output.

Petruza wrote:Please explain further.
Colors? you mean use them as actual palette indexes?

If the VRAM address register is pointing to the palette area while rendering is disabled, the color being pointed is shown on the screen, rather than color 0.

Discussed at length in the past also in even more detail.