I just tried a VERY quick wash using Secret Weapons Purple over a Bones Ghost. On the large flat hood/cowl, it almost seemed to work as a normal wash, but in areas with texture like the flowing robe, it was much more difficult to get it to adhere. After leaving it to dry for ten minutes, it wiped off very easily, even on the flat areas. However, it did seem to stain the material a bit, which might give an interesting effect if you're trying for a soft faded color.
Hope it helped.