http://wiki.nesdev.com/w/index.php/PPU_palettes wrote:
If rendering is disabled in PPUMASK ($2001), and the current VRAM address points in $3F00-$3FFF, this color will be shown on screen instead of the backdrop color. A loop that fills the palette will cause each color in turn to be shown on the screen. So to avoid horizontal rainbow bar glitches while loading the palette, wait for a real vertical blank first using an NMI technique.
I find this a bit ambiguous.
So what does it render on the screen, if the address points to $3F11 for instance, does it render color #$11, or does it render color that is in palette at index #$11?