Since nothing seems to be happening to D3675, I decided to implement this using the current state of the branch. It should make it more clear how the alert color should work in any case.

This is a convenient test to check multipaint drawing (try selecting multiple bones at the same time):

Is this calling memcmp on entire selection on every redraw? That seems like it should be avoided.

Some other minor picky things, but they can be handled fairly easily.

That structure represents the vertex group selection state, so if lattices support weight painting with Multipaint, they should use it too.

Regarding memcmp, it is unavoidable unless you want to replace it with a checksum comparison or something. On the other hand, there is just one bool per existing vertex group, not something like one byte per vertex; and the array is only used with Multipaint, and is NULL otherwise.

Edit: Also, if memcmp is expensive, then what do you call BKE_object_defgroup_selected_get and BKE_object_defgroup_mirror_selection? They search bones and vertex groups by name.

LGTM.

I'm not worry about the performance of the memcmp because it's a hanfull of bytes and usually only for one mesh.