1 and 5
are used, as shown in the image I posted above (if that image didn't load, it's
here). 6 is the only slot that hasn't been identified.
1 appears to be Button Shading, or some sort of shading override
2 is Button Outline and Button Pressed Color
3 is Button Color and Button Pressed Outline
4 is Button Highlight, but also appears to double as Button Glow
5 appears to be a second Button Glow/Shadow, though it is faint. It may appear to be stronger against light backdrops though, haven't tested this.
It doesn't appear to be as simple as rearranging to GBR, either. Due to the way colors are blended, Hue adjustments may be required in some cases — specifically for Color 2 and Color 3.
In these shots (also uploaded
here), compare the color of the Folder Arrow button to the Close button. This also demonstrates the effect that Color 1 has on the button while unpressed.