weights not showing in weight paint mode So I have a mesh that's touching another.
I gave it a bendy bone -- and following this tutorial, I added two bones at the head and the tail of the bone that I used as "custom handler references" -- then parented the bone to the mesh "With automatic weight".

Works great, except the base of the mesh lifts off the mesh it's touching for some movements.

I thought this'd be easy to fix, I'd simply weight-paint it in such a way that the bendy bones don't affect a very small portion of the mesh where the two meshes are connected. That way, it would still bend and flex as I want it to but without breaking connection to the other mesh.

Except when I go into weight paint mode, the mesh is all black.

Right, so I have since figured out that the mesh is all black because the light is behind it.

So let's pull the light in front.

Now, the mesh's no longer black and its texture is shown. BUT still no weights.

I tried and I can paint weights. I just cannot see the weights nor what I draw, which renders its usefulness somewhat ... limited.

I clicked through all viewpoint shading options and none of them seem to be helpful.

Using Blender 2.79b.

As far as options go, 

"Show Zero Weights" is set to "None"
"Restrict" is off
Ctrl+A didn't do anything

"Custom Weight Paint Range" is off


EDIT: Minimal Working Example
In weight paint mode with Viewpoint Shading set to solid, this mesh will look all grey instead of showing the weights.
Ok, I think I have this figured out...

If I set the viewpoint shading mode to solid and THEN enter weight pain mode, the weights are being displayed.

However, if I enter weight paint mode first and THEN set the viewpoint shading mode to "solid", everything is shown grey, instead.

So the easiest way to solve this grey stuff is switching to object mode and back into weight paint mode.