puNES is an open source, high accuracy NES emulator.
In addition to the standard color pallettes used by most NES emulators, puNES uses a color palette derived from an actual TV.
It is ranked as the most accurate NES emulator. However these test ROMs, unfortunately, don't test everything that matters for accurate emulation (and in some cases, test stuff that doesn't matter for emulating any games). So the test results may be a bit misleading. That does not mean it is not a high accuracy emulator.
With the release of version 0.98, FHorse released the source code for puNES on GitHub.