Solvent based inks were originally developed to mimic stained glass. Therefore, the ink is at its most vibrant when applied to a smooth, high gloss surface and allowed to dry naturally. When applied to baked pearl clay (I use Premo! pearl), it really comes alive but it is also the most difficult to control:
If you use the ink on wet polymer clay (I use both Fimo soft and Premo! white) you have more control:
You can also tint the raw clay with ink. I dribble the colors onto the raw white clay and work the clay to get look/color/pattern I want:
Or, after baking, let the inks run and do what they please!