If you’re using the Adjustment Brush, you may notice that it shows a color overlay on top of what you’re painting on. By default it’s red, but if you’re painting on something on the photo that’s red it can be a little hard to see.
What’s nice about it, is that you can change the color. Just go under the top menu to Tools > Adjustment Brush Overlay. You can change it’s color right there or use the keyboard shortcut to cycle through the colors.