Script-Fu -> Waterpaint

FU_artist_water-paint-effect.scm

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This is a script with a single adjustment which lends a soft, watercolor effect to an image. Latest version also allows changes to be left on a separate layer (if you choose not to "flatten", so the results can be "toned-down" as needed. (See lower example)

Watercolor Dialog

Original Image

Detail from "the Lovers" by Pablo Picasso

result:
Default Settings

layer dialog:

to adjust results of watercolor script

Since watercolor can create a new layer and leave it -- you can adjust the layer opacity to tone the effect down as needed. You can choose from the image menu:
WINDOWS > DOCKABLE DIALOGS > LAYERS
(or Ctrl + L)
There you can set the layer opacity. In the example I drag the slider to 50% . . . the "Watercolor" layer was already selected.