Hi Warren, just repeating what Trevinii said, try a different and
highly contrasting colour in another layer when you mask. Brown, for example, does not contrast well with your example image.
I usually have a few contrast layers, and switch visibility between them when masking - one white, one black, and possibly a couple of colours (pale green and yellow most commonly, but blue can sometimes be useful).
As Trevinii says, these layers are only an in-process guide for masking, just switch the visibility off or delete them before exporting your final masked image layer.
For example, here's your sample image with a pale green layer behind it, showing more clearly that there's a little more masking work to go.
Turn the pale green layer visibility off before exporting your image to a PNG.
Regards, Myles.
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Simplicity is a great virtue but it requires hard work to achieve it and education to appreciate it. And to make matters worse: complexity sells better. -- Edsger W. Dijkstra