The Syntax Highlight preferences panel is opened from menu → → +Editor+Syntax Highlight+Elements/Attributes by Prefix
One row of the table contains the association between a namespace prefix and the properties to mark start tags and end tags or attribute names in that prefix. Note that the marking mechanism does not look at the namespace bound to that prefix. If the prefix is bound to different namespaces in different XML elements of the same file all the tags and attribute names with the prefix will be marked with the same color.
You can edit the following color properties of the selected token:
The Foreground button opens a color dialog that allow setting the color properties for the selected token with one of the methods: Swatches, HSB or RGB.
The Background button opens the same color dialog as the Foreground button.
You can choose that only the prefix to be displayed in the chosen color by checking the Draw only the prefix with a separate color option.