In the picture above I have two feet. The blue one is the one I have selected and gone into weight paint mode. I'm trying to change some vertices to red in weight paint mode but don't really understand how to do this. When I use the brush on the foot, nothing happens. It simply remains blue. It also doesn't show me vertices. It appears solid. If I enable wire frame mode, it shows all the vertices but doesn't allow for selecting certain vertices.
How can I paint specific vertices along the seam of my foot red?