What's so hard is that the original NES does not process RGB signals. Instead, it generates a video signal directly in the composite domain to be decoded into RGB by the TV's NTSC decoder. This causes a bit of a problem, as a lot of TVs don't decode NTSC according to spec; some models put more blue into the signal to make it look better on the showroom floor, and some models (especially those for the Japanese market) warp the decoding to try to make flesh tones look better-than-real.
A couple relatively rare Famicom versions (e.g. Famicom Titler and the one built into a TV) generate RGB signals directly, as do PlayChoice 10 arcade boards. Some of the colors in these palettes are so off that developers of some NES games rereleased in arcades had to change a few colors here and there.
MottZilla wrote:
Nintendo did the same thing everyone else did though. They came up with RGB values they liked and went with them. They aren't official though.
Moreover, the palette in the acNES emulator, used by e-Reader, Animal Crossing, and Classic NES Series, is gamma-corrected for the GBA's comparatively dark screen, not the more-or-less-sRGB display of a TV + GameCube + Game Boy Player accessory or a DS Lite. The palette in Virtual Console for Wii is probably closer to Nintendo's intent.