You can achieve some measure of color with a torch and a few drop of cold blue...You can also damage the part if you go too far.

Turnbull Restorations does beautiful bone charcoal color case hardening.

A third alternative, would be to carefully polish the plate to 600 grit length wise, this will leave color in the bottom of the engraving and a pleasing gray to the rest of the plate. Follow up with clear, baking Lacquer.

My $0.02

Jim