Nah, just make dual master keys do nothing. Great idea, trust.
But in all seriousness I'd love to see it make a positive and negative copy of the door (a dual copy of the door) that can be spent with either positive or negative amounts of the costs.
If you wanted it to be extra cursed, it could result in the spend colour having a dual counterpart or something, too (e.g., you have 5 orange keys and -3 cyan keys, and you open a orange door that costs 2 cyan keys and has a dual copy, leaving you with 5±2 orange keys. This lets you open orange doors with up to 7 cost, or exact doors with 3 or 7 cost. If you had 2±4 of a key, that would let you open doors of amounts between -2 and 6 (not blanks of course), or exact doors with -2 or 6 cost. If you had 2±2 keys of a given colour, you could open blank doors or doors of up to 4 cost of that colour, or 4 exact doors (0 exact doors are just blank doors lol)) (Yes I know making it result in a dual counterpart doesn't make sense with the rest of the way bi-coloured doors work, but it'd be cool nonetheless.) (Don't ask what happens when you whip out a master key when you have 1±1 master keys. Just make it ±(1±1) copies or something. What if you have 1±(1±1) master keys, you ask? Uh... ±(1±(1±1)) copies. Yeah, just make it recursive. That surely can't end badly!)