That doesn't really matter, you don't really need to convert between different color spaces that much. Whatever the user specifies you convert into one color sp

That doesn't really matter, you don't really need to convert between different color spaces that much. Whatever the user specifies you convert into one color space, say CIE XYZ or whatever suits the interpolation you want to perform, and then you convert the interpolation result into whatever you need as the output color space. The only real requirement is that the interpolation space covers all input spaces, then it does not matter that not all input spaces cover the same colors or have non-unique representations.