To get that intense red on the copper, simply apply white paste
flux to the area that you want to turn red. Next, heat the copper
slowly and you will see little areas of oxidation begin to form
and gradually grow to cover the entire area where the paste flux
has been applied. Varying colors can be achieved depending on how
fast and how hot you heat the metal. Let the piece air cool.
Finally and most importantly, don’t pickle the copper. Instead,
soak it in some warm water with a little baking soda. This will
remove the flux but not the oxidation. The resulting bright red
oxide surface is very durable and when waxed or lightly buffed
with green rouge, it looks fabulous. Hope this helps.