Hrm, I'd dare say the flavor for Colour Change implies that it's possible.
This pokémon is adept at rapidly changing the color and texture of its skin in order to blend in with its surroundings. Not only that, but it can alter the chemical composition of its skin to match the properties of any sort of energy that it touches. This response is involuntary when the pokémon in question undergoes pain, and therefore it will change its type to match the type of whatever damaging move by which it is struck.
So, I would assume you could voluntarily change types if you have the Colour Change ability... and Curse generates ghost energy, doesn't it?
This pokémon is adept at rapidly changing the color and texture of its skin in order to blend in with its surroundings. Not only that, but it can alter the chemical composition of its skin to match the properties of any sort of energy that it touches. This response is involuntary when the pokémon in question undergoes pain, and therefore it will change its type to match the type of whatever damaging move by which it is struck.
So, I would assume you could voluntarily change types if you have the Colour Change ability... and Curse generates ghost energy, doesn't it?