The following illustrates an error in the Color Pickup/Color Refresh of the custom brush.
Explanation:
The brush was set up very basically, 100 opacity, 100 loading, 100 size, no tapering, no grain amount, no grain in the custom brush, no jitter, no rotation, and no pressure, color or luminance variations.
Brush head is default that comes with AR "Artistic/Droplets"
Dab spacing set to 100%
Color Pickup 25%
Color Refresh 100%
White and black strokes on a layer with a flat dull magenta fill.
As can be seen, even though the each dab should slowly be at a color closer and closer to dull magenta, the edges of the brush dab (i.e. where the brush head values are neither white nor black, in the source or created during the scaling to 400x400 of the brush head) show less of a color pickup and more of a color refresh, leading to an imbalance which creates clear, and artificial, outlines of the brush dabs. This error, which is clear here, has subtle effects when using lower dab spacing but it is still quite noticeable.
For clarity, the image was taken with canvas lighting turned off.