I have a NES, it is working normally, but one of the game colors appears as black. In the game Mario Bros 3, the name is in black, some blocks are black and the goombas too. What can it be?
Because the NES cartridge port uses a separate bus for fetching graphics data versus CPU code, dirty cartridge connectors can often cause graphical anomalies without otherwise affecting cartridge behavior. If the lowest-order address pin to the cartridge was dirty, that might could cause some colors to appear incorrectly for some shapes. If the cartridge saw the pin as always seen as reading low signal, colors 1 would become color 3 and color 0 would become transparent. If it always read high, the roles would be swapped. If it's dirty, however, then it might usually read correctly except when the state of nearby pins is the opposite of the correct "address 0" state.