I would like to modernize this fireplace in a 1970s split level. I’m interested in applying a German Schmear, but I am concerned that the black mortar will bleed into the white mortar casting it yellow. I cannot find a definitive answer to this concern.

Anyone out there with experience on mortar colors and how white mortar would respond to black mortar underneath?

If the German Schmear technique won’t work, I will likely paint the whole thing a very dark gray to match the mortar.

Thanks!