I also tried boiling the PVC parts in a dye/water mix - this resulted in minimal colouration to the PVC, but they did shrink!!
I then tried adding the priming fluid to the boiling water (warning - this can be dangerous and the fumes are not something you want to breath in !!!) - again, this resulted in minimal colouration to the PVC.
Both of these things were copied from the Internet :rolleyes
More research required
I'm not sure why the dye won't mix with the priming fluid - it sort of beads on the bottom, like adding water to oil :confused: I suspect I need a different type of dye or a different type of priming fluid.