When you use the Colour Gallery to drag a Named Colour from one Xara document to another if the colour name already exists you are given 2 options:

Match: The colour being dragged becomes the same as the colour already in the target document.
Don't Match: The colour gets added to the target document but it's name is altered by appending a/b/c etc. to the end of the colour's name.

I like to see a 3rd option:

Replace: The colour in the target document is changed to the one being dragged.

I appreciate this would not be possible in situations where either colour is linked to another.