Lori is correct.
a scumble can be any color(transparent,opaque, semi-transparent, semi-opaque) painted on with a dry brush.
however, technically, one could also glaze with opaque colors, right?
An interesting effect to try would be to paint using a medium with damar varnish in it, then after it's had a month or two to dry(or maybe not), then "scumble" with turpentine. That will get some unique effects.
I was referring to the what was traditionally a scumble... as opposed to a sfumato or sgraffiti.