Food coloring (blue or green) will work in floor polish.
--- In Homebrew_PCBs@y..., "Karlis" <krahabors@h...> wrote:
> Just check the best color before.
> I had few of them, and the blue and black was much better then
others. The
> green one washed off immediately.
>
> KPL
>
> > Yes I tried to work with permanent markers, and I had good
results. I used
> an OHP marker and it worked fine with Ferro chloride.