You can never ask too many questions Henrik , that's what forums are for. I have never painted Scalextric track but it has been done although adhesion is a bit problematic. Experimenting with different paints on a scrap piece of track is a good idea. One word of warning - as you have a digital track be very, very careful when doing the crossover sections, paint in the working parts will not do them much good.
(This post was last modified: 1st-Sep-19, 04:15 PM by CMOTD.)
The white wall paint at some places on my Scalextric Classic track is still in use with good adhesion to the track.
Before painting, it is important to degrease the track with some white spirit.
I have also used grey turpentine based primer, which did the job too.
To get a 'speckled' effect you only need to rub gently with a cloth over the still wet painted surface (see photo).
rallyhub
The white wall paint at the Scalextric Classic track. The speckled effect on the grey painted Scalextric Classic track
Got 20 % discount of the wash, primer and paint cause the shopowner haven’t had the questing “what should I use to paint my slot at track with” in his 32 year career ?