weight paint problem?

HI, I am trying to rig a hand and it my first time using weight painting.The weird thing is I pick the mesh in pose mode and the bone.When I go to weight paint mode I can paint the first area of mesh, but when I pick a new bone the red disapeares on the first area and I can’t paint anymore at all.What the heck am I doing wrong?? Thanks.

The red should disappear because the bone it’s associated with is no longer selected. I can’t think why, when you select another, bone you can’t paint because even if that bone has no VG it will make one as soon as you start painting. If none of this helps you please post a .blend so we can look at it.

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Figured out the problem. Duh!! I had the wrong name in the OB box for the armature modifier. Thanks for the answer though.