Documentation below, I've not found anything that actually uses it though so maybe this doesn't really matter
4000002h - Undocumented - Green Swap (R/W)
Normally, red green blue intensities for a group of two pixels is output as BGRbgr (uppercase for left pixel at even xloc, lowercase for right pixel at odd xloc). When the Green Swap bit is set, each pixel group is output as BgRbGr (ie. green intensity of each two pixels exchanged).
Bit Expl.
0 Green Swap (0=Normal, 1=Swap)
1-15 Not used
This feature appears to be applied to the final picture (ie. after mixing the separate BG and OBJ layers). Eventually intended for other display types (with other pin-outs). With normal GBA hardware it is just producing an interesting dirt effect.
The NDS DISPCNT registers are 32bit (4000000h..4000003h), so Green Swap doesn’t exist in NDS mode, however, the NDS does support Green Swap in GBA mode.
Documentation below, I've not found anything that actually uses it though so maybe this doesn't really matter