An AnsiColour created from the given colour.
static struct RGB { ubyte r; ubyte g; ubyte b; } assert(RGB(255, 128, 64).toBg == AnsiColour(255, 128, 64, IsBgColour.yes));
static struct RGB { ubyte r; ubyte g; ubyte b; } assert(RGB(255, 128, 64).to!AnsiColour == AnsiColour(255, 128, 64));
isUserDefinedRgbType
Converts any suitable data type into an AnsiColour.