[Harry K.]

Not sure what you've tried, and I'm not a user of Gimp but from what I remember the toolbars are pretty similar.

Is there 2 little squares with black and white in, probably on the same toolbar as the brush tool? Like pictured https://i.imgur.com/2Z2qGDQ.png
If you have already tried that and it doesn't work, does Gimp support layer styles? You could add a color overlay to the layer you applied the brush too. This should change the color of the brush.

For layer styles it's usually double click the layer and look for "color overlay"