I also need to do it with the mixer brush, but i worked out you can just make your own brush with a rectangle.
One thing that's strange with that is that if i make my own brush, then scale it down a lot (using [ & ]), the color it draws becomes lighter. I guess that's because the brush scales in thickness as well as length.
Would anyone happen to know how to prevent this? Either by forcing a brush to paint only one solid color, regardless of brush scale, or to restrict brush scaling to only one dimension?
»One thing that's strange with that is that if i make my own brush, then scale it down a lot (using [ & ]), the color it draws becomes lighter.«
When scaling a Sampled Brush tip you have to expect softened edges (if any) due to the resampling, but that should not make a Stroke »lighter«, so could you please post a screenshots taken at View > 100% with the pertinent Panels (Toolbar, Layers, Brush Settings, Options Bar, …) visible?