If the lettering is quite large you may be able to paint the text. Lightly wet and dry to give a sharp edge. Silicone spray over lettering. Fill it with something soft like plasticene. Clean off the silicone overspray, spray yellow and pick out the plasticene.
I would have painted the metal yellow before engraving, then painted the lettering with a paint that wont react or bind with the yellow, water based maybe and just wiped the excess off.