Also, keep in mind that the colors you visualize in NESmaker will not render exactly the same when running on an emulator or even a real NES console.
Every system interprets the indexed colors differently, if you play your game in the FCEUX emulator the colors will be different than if you play it in MESEN. Same when playin a real console plugged on 1 TV, or on another TV... or another NES clone...
In FCEUX and in MESEN emulators, you can change the palette system in the options, to find the one that suits more to your design. (Personally, I prefer the colors of fceux)