Win 7 64 Bit
Nvidia 1060 GTX
(example: 2.79b release)
(example: 2.80, edbf15d3c044, blender2.8, 2018-11-28, as found on the splash screen)
Short description of error
Brush colors flip menu item is greyed out in texture painting mode. In Blender 2.79 this menu item is fully functional. And in vertex paint mode the brush colors flip tool works too.
Exact steps for others to reproduce the error
Based on the default startup or an attached .blend file (as simple as possible).
Switch to Texture Painting, and add a texture and a material.
Have a look in the Brush panel for the brush colors flip tool.