Opinions vary, but if C-Track isn't ballasted then you really need to find a colour matched ballast if you are just looking to run some down the sides to blend in the scenery, without having the track look like it is sitting on top of the scenery. IIRC Busch do supply a ballast that is colour matched to Marklin C-Track.

However if a different colour ballast or track bed is desired then one needs to either paint the C-Track, or ballast all over it. It probably doesn't need more than a bit of a sprinkle between the rails to have sufficient colour change to get away from the plastic colour of C-Track.

Some people find the sides of C-Track to be rather steep as a ballast shape, and so will ease that by blending the slope into the scenery so shape of the sides isn't so steep. It can also look much better with the sides of the rails painted with a rust colour instead of being a shiny colour of newly laid prototype track. It doesn't need a lot of paint, and if the paint is really 'wet' it is possible to overdo it and it doesn't look right. Real world track beds will also have a spread of a rust film over it from the wear and tear of passing trains, so it is not unrealistic to airbrush to complete track bed close to the rails.

It is worth going out to any rail track and just look at how it has weathered since the last load of ballast was placed. If you can find a spot where some fresh ballast has been laid you can really see the difference of the natural weathering. It all depends on how much effort you wish to put into your layout.

No need to choice of the color ballast when you can cover the C-tracks by use special plastic glue for the ballast.
I did planned to test some C-tracks but found out unnecessary waste of time since Märklins C-tracks are expensive and load.

In recent years Marklin have been advocating (via their books) the ultimate layout would be a hybrid of C and K track. Here is one such book and I enclose some photos showing their ideas, in some cases the C track is painted and ballasted and laid on top of the scenery in first few photos, but elsewhere in the book they show the C track connected to K track and cut inot the scenery and ballasted and painted to match the K track, and in some photos its hard to tell which is which, and clearly a lot of time (and money) has been spent to match. Merkur who make the ballasted trackbed for K track also used to make loose ballast to match for such occasions.












There are other examples on this forum of modules built with C track which have been ballasted and look great. Such as this one






I hope these give you some inspiration and ideas

BR