The problem is a combination of the lack of contrast differences (you should use dark for one end and get progressively lighter, rather than having dark colors on both ends) and the Purkinje effect, which changes how the human eye sees color at night (basically, we don't).

At night, our eyes respond better to blues and greens than reds. So the purple means you're getting red and blue light. In dim light, that red light isn't going to even register, so you're basically looking at the difference between dark blue and black, which isn't much.

A camera, by contrast, perceives color roughly the same regardless of light level, so you should be able to see the purple lanyards when screening the photos even if you can't see them with your eyes, assuming there's enough contrast. However, even there, you may see differences in sensitivity between the color channels, particularly if the camera has a built-in UV filter, which may reduce sensitivity in the blue end of the spectrum.

To answer your question, though, you should pick colors that are highly visible and distinct at night. This could include stripes, glow-in-the-dark material, phosphorescent material that will emit light in the presence of black light, etc.

I would probably go with white, black, and a light to medium green. In terms of visual sensitivity, your eyes see green to greenish blue objects most easily. This is true without regard to color blindness, as the rods that pick up light at night are not responsible for color vision. And choose the order of the lanyard colors carefully, too. Make black be "don't care", white be "don't photograph", and green be "ask". That way, the ones that are least likely to be seen are least likely to matter.

Another problem, of course, is that your black dye could very well be really dark purple, but that's another issue for another day. :-)

Also related:

Why are sensors less sensitive to blue light?