From what I've read, the problem is that unlike all the other, more modern consoles, the NES doesn't use RGB at all to render its graphics. Instead, it generates directly the NTSC (or PAL depending on the region) signal displayed by your TV, which is why the same game would have different colors when played on different TV models and/or with different Tint/Color settings.
Apparently, that's why there's no way to create an accurate RGB palette, since the NES uses a totally different system. It's even harder than matching the colors between a document on screen (an RGB process) and printed (a CMYK process) which both use a different color rendering method and different color space.
Your best bet would be using an emulator with accurate NTSC composite emulation, which I think at least Nestopia does. But if you do, say goodbye to well-defined pixels and hello to fuzzy graphics with artifacts. What was made to look good on TV isn't so hot on a computer display.
As for myself, I use FCE Ultra's default palette. It looks great, and I wouldn't notice if a few colors were slightly off from what I can remember from playing on a real NES a decade ago.
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Current project: Castlevania: Aria Of Sorrow (GBA)
Upcoming project: Castlevania Legends (GB)