Hello, Gaius ...
As you know, Playmobil produced Color Paint Pens (sets 3600, 3601 and 3602) to be used on white Klickys.
Paint pens are not as easy to use as a paint brush. It is also more difficult to get a consistent even color with paint pens. Also, the paint pen colors tend to fade unevenly over time.
Most art supply shops carry different types of paint pens from a variety of manufacturers. And, yes, there are white paint pens (also gold, silver. etc.) Paint pens seem to be a bit more expensive than paint for brushes. It is also very important to keep the tips clean and shake them well before using.
If you are looking for a different painting technique, you might want to try airbrushing, and use paint pens for the fine detail.
Good luck and all the best,
Richard