I use them quite a bit with a few different techniques. The purple shade is very dark, so if you're applying it over slannesh grey it will be very difficult to make it look even. I'd recommend thinning it down with lahmia (sp?) medium. 2 parts medium to 1 part purple shade sounds about right for the colors you're using. Don't use water to thin shades. They have a tendency to turn white if you mix them with water, and they won't flow as evenly. Thinned down, you should be able to slather it on and get decent results. Just look out for excessive pooling. You can also try applying it selectively to only the recesses, though you would still want to thin it first.