Recently the Weight Paint Mode got an update with the Weight Overlay method. Instead of overlaying the weights they are multiplied when using the slider. When they should be flat shaded, the user can instead switch the shading mode to flat and the same result is achieved.
This should also be the case for painting in Vertex Paint & Texture Paint. Especially in Vertex Paint Mode it is incredibly hard to paint accurate colors when you have to always chose if you want to see the shading or the color information.