Pete, it's not colors only, it's protection from corrosion. If you delete case colors, the surface of steel will be open for corrosion. That's why surface has to be sprayed with trasparent laquer. The idea of French grey - surface is protected from corrosion just after quenching in water solution.
I heard gunsmiths in Ferlach use argentation to imitate and to "restore" French grey finish with yellowy patina.

Geno.