This was discussed a bit in bug 547004. Some ideas in bug 547004 comment 20 (talking about attachment 437574 [details]): "By the way, the colors in the bottom of the color picker window would represent the 'autocomplete' attribute. In other words, it will show last used colors." but this was at the time we didn't thought about using native color pickers, and not all of them have a "custom colors" part (Windows' one has, Gtk doesn't have something similar). In bug 547004 comment 71: "This can also be a drop-down list of colors, or some "custom colors" added to the color picker when possible." but the "drop-down list" might be use for list attribute (bug 960984).