Those are image files, that you can not adjust by setting an rgb value for.
They are small .png images, which are base64 encoded to go into the xml.
You uselessly find them at the end of the xml skin file, and are named such as:
<element id="FormBorderTop">
<element id="FormCloseButton">
followed by a lot of 'garbage' characters. Searching the forum should help you top learn to create those yourself if you want to.
It is not possible to edit .xmlc skinfiles. They have bitmapped elements for the player panel, and are compiled (hence the 'c')