I used the Stampin' Up! Pigment Sprinkles as my watercolor. They kept bleeding through (sometimes even through the white heat embossing--especially the reds.)
Things have to be really dry between steps, so it takes some patience.
When you add the white ink, magical things happen with the watercolor underneath! I ruined my white ink pad because I forgot to wash my stamp between each stamping, but I don't care. I LOVE this technique.