'UsePartColor' checked on new Unions if monochrome

[Mister_Mystery]

I’d like it if the UsePartColor was automatically set to true on new unions if the union only has one color. It would be easier than manually setting it each time and it makes sense that a union would be default colorable like a normal brick without having to change anything. Of course, a union that has multiple colors would have it set to false by default so that the original colors are preserved.

[NWSpacek]

Thanks for adding this!