2.1.691 Part 1 Section 18.8.3, bgColor (Background Color)

For additional notes that apply to this portion of the standard, please see the notes for color, §; tabColor, §

a.   The standard does not specify the priorities of the various color types.

Office prioritizes the attributes as auto < indexed < rgb < theme, and only roundtrips the type with the highest priority if two or more are specified.