Why is it that, when I open a rig that has the limbs separate and the bones don't move the mesh yet, I select the armature and go to pose mode, then open the rig of the body in weight paint mode and try to select a bone, the whole armature is outlined in red rather than outlining a single bone in blue?
First check that your armature is in Pose Mode. Switch it to Pose Mode (Ctrl+Tab) and then select your mesh and paint.
If it doesn's help, you probably didn't assign Armature Modifier to your mesh. In this case select your mesh, then holding Shift select your armature and press Ctrl+P and then choose Armature deform.