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I've done a logo with a green watercolor look. I would like to replace the green with a reddish orange.
I'm seeing that you can replace a solid color using the FX Stack, but is there a way to replace the color in a textury watercolor image with another color?

As long as you keep a single colour on a separate layer you can: set "preserve layer transparency" and paint, or draw a coloured rectangle over it.

You also have the Colorize and Tint blending modes for layers and/or tools, experiment a bit with these...