Why not make this an option (can be on by default), since there are times where it's useful to see the shading while weight painting, otherwise you can't see it's shape and bad deformations/distortion isn't as visible.

Simple before/after example, full reply here: T73434#1204210

Not sure if this effect is desirable, but I would do something like this in the vertex shader:

/* Saturate the weight to give a hint of the geometry behind the weights. */
vec3 view_normal = normalize(normal_object_to_view(nor));
vec3 light_dir = normalize(vec3(0.0, 0.5, 0.86602));
color_fac = max(dot(view_normal, light_dir), 0.0);
color_fac = 0.1 + (0.9 * color_fac);

This has the following effects:

• The shades gradient simulates a light coming from a slightly upper angle instead of from the front, which integrates easier with other studiolights and matcaps. It also avoids accentuating shape contours too much.
• Prevents completely dark areas and reduces the contrast of the gradient, introducing less distortion into the actual color data.

Current Patch:

Proposed:

I added the arbitrary light with some small adjustments so the light is also visible from the inside of the mesh.