I downloaded and installed the environmental brush pack by IForce 73 and I love it. I am having a problem that I just cant figure out and need some help. One of the brushes is the HNT Bark highlight brush. It makes a two color mark that is black and white. I am trying to change it so that the two colors used are from the background and foreground colors and not the black and white colors used.

Any help would be much appreciated.

That's an unusual brush tip in that it's an RGB/A image with what looks like a greyscale content but actually contains non-grey colours. Because of that, it behaves as a coloured stamp (with the colours being mostly black/white/grey). As such, the selected colour has no effect on it. It's a 'picture stamp'.

If you convert it to a true greyscale content, it behaves as an 'ordinary' brush tip and paints in whatever colour you've selected, opaque on the left, less opaque on the right. I don't know of a way to have any colour except the foreground colour be painted by a brush (except for picture stamps). There may be a method I've missed and if so then I hope someone can give advice.

As a very rough workaround, you can toggle the foreground/background colour between strokes while painting to see if that gives a useful effect.

If you like the effect of that black/white picture stamp and want it in different colours, you'll have to make your own bi or multi-coloured picture stamp brushtip. This is technically easy but making a good looking or useful one needs skill, experience and patience.