Let's take the example of Ecuador spindle whorls. They were made from black clay, and some have remarkably intricate carving. However, because this is "black-on-black" the carving is not easily seen (without a hand lens). So here's a solution I was told in the '70s, that I have recommended many times, and was used with great success. Rub toothpaste into the carved areas (!).

It's very easy, readily available from almost any store, it's cheap, and it works. You rub it in (as carefully as you're willing to be), and let the whorls (or beads) sit for a week. The toothpaste will dry out and become reasonably hard. And since the carvings are recessed, the toothpaste is not worn-away by handling.

I see no reason why this would not work with Venetian meander beads—or any beads that have white lines that have fallen out. (This is a fairly common occurrence, because the black and white glasses are not entirely compatible.)

Good luck. Jamey