05-30-2022, 09:58 PM
A grayscale image (truly grayscale, one single value channel, so Image > Mode > Grayscale) works like a mask (inverted): white is transparent; and black assume the foreground color. A RGB brush (all three channels even if R=G=B) is a color brush, and Gimp uses its color, so the color is not changed by the ground. The clipboard mask being a mask is natively a grayscale image, so uses the foreground color.
But... it appears that the "clipboard brush" is always seen as a color brush, even if grayscale. But if you export your 2px all-black brush as a GBR, it will work as a mask brush.
Experiments also show that the slight difference that you see between the two can be ascribed to the color/mask difference, because the exported 2px brush behaves exactly like the clipboard mask brush.
But... it appears that the "clipboard brush" is always seen as a color brush, even if grayscale. But if you export your 2px all-black brush as a GBR, it will work as a mask brush.
Experiments also show that the slight difference that you see between the two can be ascribed to the color/mask difference, because the exported 2px brush behaves exactly like the clipboard mask brush.