White ink , fine tip, felt-tip pen ?

Sharpies are permanent. I don't know about European distribution, but in
the U.S., they're as common as pencils.

 

Most are. In my experience, the really fine point Sharpies use an ink you
can rub off with your fingers.

 

In the UK gel pens by Mitsubishi Pencil Co are half way there.
www.uniball.com apparently (flash junk)
I just tried one.
The bright pink one is good colour contrast and will write fine lines on
black plastic. If you allow a day for it to dry then it is reasonably fast,
you have to rub it firmly before the dried gel will rub off.