Coloring & Oxidizing Silver

Hi all, I read through a thread on Coloring & Oxidizing Silver in the
archives but couldn’t find an answer to my question. I’m doing some
silver/copper overlay pendants and want to color the top silver
metal with liver of sulpher to a blueish/black. However, I want the
copper underneath to be treated perhaps only once or not at all and
then stayed. Can I coat the image area with clear nail polish and
then continue to dip the piece? If so, can the polish remain once I
later laquer the whole piece? What would folks recommend? Thanks in
advance! Diana