Not all cobblestones are dull or shiny grey. It is definitely interesting to add some colors to cobblestones. And this was what I did for my layout. One section of my Loewenherz railway park is covered with cobblestones. These are finely engraved using only mechnical pencil.

Priming the Cobblestones

I primed the cobblestones with diluted Vallejo Model Air Cork Brown and Sand Yellow mixture with lots of thinner.

Individually Painting the Cobblestones

After letting the primer dry, I individually and randomly (yes I did that) each cobblestones with various Vallejo acrylic paints. You can see besides various hues of brown, I also used Aluminium and black grey. The aluminium paint gives some shine on certain stones.

And the overall effect? You got to zoom in and see it in person to appreciate the effect. I keep one section (bottom quarter) unpainted/only primed to show the contrast and bring out the effect of painting the cobblestones.

The overall effect looks great.