You might try contacting Elaztech, and asking them what to use.
If you want to take a "do it yourself" track, try some different stuff and report your results here, so we can all learn from you.
If I were going to try it, I'd start with Sharpies, and then Spike-It Dip and Dye.
I've always found Elaztech very sensitive to soft plastics, so I would use one bait, trying the dye on different parts of it, so I didn't ruin more than one.