To have an RGB NES seems to be the dream of many. However, the RGB PPU, the 2C03, is not without its drawbacks. Any game, and there are a few, that use the color emphasis bits of the 2C02 will show the completely opposit effect. As they dim the screen we should say that they are de-emphasis bits. We can see this effect with a 2C02 in games like Super Spy Hunter when you pause the screen or the scroll at the beginning of the Fantastic Adventures of Dizzy. With a 2C03 the Dizzy scroll becomes partially becomes a white bar and Super Spy Hunter's screen turns pure white.
Now this doesn't really effect gameplay, but I know of two games that are entirely unplayable because of the way they use the color emphasis bits. The Euro-only game Noah's Ark from Konami uses the color emphasis to darken the portion of the screen not covered in water. Using the 2C03 you will see a blank white screen except for the water. Electronic Arts The Immortal and Acclaim's Airwolf always use the color emphasis bits to darken the palette. Playing those games on a 2C03 would be an exercise in futility as you would always get a pure white screen.
There are many games that use the grays in the palette which are non-existent in the 2C03, but you could consider it a day to night feature. No good game uses those grays to such an extent that gameplay is affected.
Now this doesn't really effect gameplay, but I know of two games that are entirely unplayable because of the way they use the color emphasis bits. The Euro-only game Noah's Ark from Konami uses the color emphasis to darken the portion of the screen not covered in water. Using the 2C03 you will see a blank white screen except for the water. Electronic Arts The Immortal and Acclaim's Airwolf always use the color emphasis bits to darken the palette. Playing those games on a 2C03 would be an exercise in futility as you would always get a pure white screen.
There are many games that use the grays in the palette which are non-existent in the 2C03, but you could consider it a day to night feature. No good game uses those grays to such an extent that gameplay is affected.