I believe this “feature” (aka. “bug”) is with your graphics drivers,
and that is indeed the buffer from your video memory that shouldn’t be shown on screen( that would be the “bug” part!).
Nope, its definitively a bug in blender. To reproduce it just create a default UVsphere switch to weight paint, open grease pencil panel (view menu) , enable grease pencil, create a new layer and select color.
Tested on two machines with different OS.
Did you update or checked the version of the graphic drivers in both machines?
it is a bug. please submit to the tracker, with the note that the color picker pop-up tool is not refreshing properly to show you its panel.
no need to report, driver bug, an optimization that doesn’t effect most games.
It’s not restricted to Nvidia cards though. Just tried it out with an ATI 4850 on linux (fglrx driver).