Hallo,

Use color as mask Option doesn't work for custom made brush.
I followed this tutorial:
https://www.youtube.com/watch?v=E-d6kmLthAk
And at 2:13 you can see he select option "Use color as mask" and it works just fine.
I did the same saved it and it doesn't work.
Wonder how I can go around this problem?

It is not working for me on the 3.0.1.1 version under Linux either. Didn't test on Windows.
As a workaround you can use the Clipboard option.
Press CTRL + A, then CTRL + C, then go to the Brush Settings and press the Clipboard button instead of the Stamp one.

Known bug: https://bugs.kde.org/show_bug.cgi?id=369532