I would like the option of white printed behind colour areas, or not, to create semi-transparent or translucent colour areas on clear stickers.

The same way it works with colours/white on holographic stickers :)

I want to design a clear cling with an opaque mark. It would overlay a dial to indicates the maximum or optimum position of the dial handle (like a clock hand), without impeding the visibility of the dial handle. As conditions change, the cling would be rotated, so it can't be permanently attached. There are a couple of these devices on the market already, but they use suction cups and fall off.