Select Colors to Enhance (SCtE) is a very handy tool, sort of like a eyedropper-mask-tool, but much easier to refine its selection.

If SCeT had an option to INVERT then the remaining portion of an image, especially if it has many colors, could easily turned transparent (or apply other effects such as saturation and hue adjustments).

If not an invert function, then a CONVERT TO MASK option. Then Xara's masking operations could be used.