Get a little too aggressive with your sander and in a heartbeat you can cut right through the thin face veneer on plywood. On highly visible workpieces, such as a cabinet top, you'll probably need to replace the damaged workpiece. But it might be worth trying this trick first. If the damaged spot is in an inconspicuous location, it might never be seen. Practice on scrap, then work on your project. What have you got to lose?
Note: This fix works only on lumber-core plywood. With MDF-core plywood, the substrate will absorb finish and turn dark, so either replace the part or patch the veneer. Test your technique first on scrap, especially when working with a dark veneer, such as walnut, that contrasts with the light-color core plies.
The grain on the layer beneath the veneer runs perpendicular to the surface layer. Use a sharp razor knife to replicate the surrounding veneer grain pattern in the substrate. Make some shallow slices and some slightly deeper.
With the spot camouflaged, spray on a coat of clear shellac to seal the wood -- brushing may lift the colored pencil marks. A coat of gel stain provides a uniform color. Create additional "grain" by lightly wiping on a second coat of gel stain over the spot, leaving light streak marks.