To paint smooth skin point weights
Reflection is disabled for the Paint Skin Weights Tool, but you can use Mirror Skin Weights (in the Skin menu Weight Maps section) as an alternative method to reflect skin weights.
When you select an influence to paint, the mesh displays color feedback (by default) in white and black. You can verify that Color Feedback is turned on under the Display heading. Turning on Color Feedback helps you identify the weights on the surface by representing them as grayscale values (smaller values are darker, larger values are lighter). You can also turn on Use color ramp under the Gradient heading to view weights in color.
For example, make sure that the Weight Type drop-down list is set to Skin Weight, and select the Normalize Weights mode you want.
The values you paint set how much this joint influences the painted vertices relative to the other joints making up the smooth body (up to the number specified as the Max Influences in the Bind Skin Options).