It wasn't that hard actually. You take the colors you want and with a rubber roller on a smooth surface (like glass). Roll over the paint until you get a gradient you are happy with. Note worthy the biggest issue is maintaining the consistency with of the gradient.
Oh man thank you so much. Trust me it is possible. Just go in with a plan that helps a lot especially when dealing with acids. The color part that is just fun and you are right one color could easily makes piece work.
No, actually it was a happy accident. After applying a coat of color the outlined happened because the plate is etched in layers as well. The windows are the deepest layer of the plate with the guy on the left being the closest to the top. It was all in an effort to create physical perspective.
Hey Sorry for the late reply. This was actually all the colors on the same plate. All the colors were spread across a glass and blended with a roller and applied that way. No post color production. Thanks for the comment.