Create a new brush in Rebelle or in Photoshop etc. Save the brush as 512 x 512 sized PNG file. Add the brush file to the ~/Library/Application Support/Escape Motions/Rebelle/Brushes or the equivalent on the PC. Restart Rebelle. Go to the properties panel. Select watercolor etc and then select the brush from the lower half of the panel. Apply brush stroke with different water, loading, pressure etc
Rebelle comes with only a few strokes, you can add new ones such as water colors etc. The brushes have to be 512 x 512 in size and PNG format. You can create the brushes in numerous ways such as in Affinity Photo or Photoshop or even Rebelle
There is no documentation for the brushes so a lot of it is trial and error. You can see from the image above there are a number of different brushes added to the original four brushes.
Create document 512 x 512 and fill with black and set the brush (in Photoshop etc) to white and then apply dot brushes or a splatter of dots, save that PNG file to the Rebelle brushes folder
You can also use other images such as type, diamond designs, spots, photos of people etc. Not all brush strokes are effective if applied as a single stroke. If just added as a dab the brush becomes far more effective as a watercolor brush etc. You can then modify the wetness / drying etc as required for the newly added strokes