Quoted from zacaj:
Sadly I couldn't find any cheap sources of stranded wire online, so I came up with this. Tried it first with a regular sharpie, but the ink would rub off quickly, so I got some oil based ones, which seem to stay on the wire well once they dry. Stick a marker in the top, pull your wire through the side, and you'll get a messy but usable stripe.
You should consider posting this in the "Show and Tell" thread about homemade tools - https://pinside.com/pinball/forum/topic/show-and-tell-your-diy-homemade-tools
I think a lot of people there would find this interesting. I certainly never thought of this.