Practicing White Clayboard Engraving

I'm having a little love affair with clayboard engraving (scratchboard.)  I started in the traditional black and have now moved to colorized white clayboard.  This satisfies my need to create in the most tedious, time-consuming medium available.  Actually, I love the detail that can be achieved.

Right now I am experimenting with ways to colorize the clayboard, which is a very smooth surface and doesn't take paint very well.  I've tried watercolor, acrylic, ink, gouache, and oil.  Anything opaque isn't making me happy.  I like the luminosity that a transparent color can provide.  Oil, gouache, and straight acrylic is too thick to scratch well.  Watercolor looks nice but smears when re-moistened.  Ink stains the board and is luminous enough but is soooooo tedious to work with because you need to build up layers of color so gradually to avoid blooms. 

Here are two ink-colored boards I've done recently.