Highlighted Maker Marks ?

[JLM]

Being new to US edged weapons, I've notice in some references that the maker marks and firms are sometime highlighted in white on a blade. Can someone tell me what is used to do this ? Is it a chalk or ordinary white paint that is wiped over the recesses in the letters and numbers that create this effect to make it stand out ? Any suggestion on what to use, or whether to do this or not. I often see P O8 luggers, that are unit marked, exhibit this same characteristic.