In real life track ballast forms the trackbed upon which railway sleepers are laid. It is packed between, below, and around the sleepers. It is used to bear the load from the tracks and trains, to facilitate drainage of water, and also to keep down vegetation that might interfere with the track structure.
In model form we use it to simulate the appearance of the real thing and it does nothing structurally with track laying.
Different sizes and colours of ballasts are available for use on model railway and the choice of colour will depend on the real thing used in the area you are modelling. We stock grey and brown and they come in fine and very fine options. Works best when combining two different sizes.