The ink is being mixed with the plate oil with a small pallett knife. The plate oil makes it more workable and smooth.
This is when you can mix colors and test how they look.
Take your time mixing and blending the ink, make sure it's mixed well.
< It helps if your working environment is warm, the ink mixes and flows into the lines of the engraving better. It is also more easily wiped.
Want to see this copper plate in the several stages of being engraved before printing?