Is it okay to use varnish for imitation copper painting?

Of course. It is ok to use varnish for imitation copper painting, because imitation copper painting itself is a kind of painting with dark color. If you paint with varnish, the brightness of the copper-like painting will be improved, the overall painting will look more saturated, the color will be brighter and brighter, and the overall texture of the copper-like painting can be improved, so it is ok to use varnish for copper-like painting.