Thanks everyone. The consensus is clear, once the pigments soak into the PVC they're not coming out.

A friend at work gave me the answer: Lay masking tape where the line is going to go, make the mark on the masking tape, cut along the line, then peel off the tape.

To quote T.H. Huxley's response to Darwin's first lecture on natural selection, "How extremely stupid of me not to have thought of that!"