This tutorial is for photoshop not GIMP
First go to your smudge tool
Then go to the top and click "window"
Scroll down to "Brush"
Now click "scattering"
Now change the brush to this:
Then click the "Scattering" text and pull thet op "Scatter" to 90%
Keep the "Count" at 1
Now turn smoothing on if it's not already
Now your smudge will forever be win
It will turn it from always just pulling colors to blending, pulling AND mixing it