I don’t want to start any rumors, but I have it on good authority that the adjustment brush tool gets around. Even with the Auto Mask option that is supposed to keep the tool from crossing boundaries, you never know how much of an image it has touched. That innocent local edit could be plastered all over, and you might not even know it. Except, of course, that Lightroom provides a handy feature called mask overlay that will tell you exactly where your brush has been.
One way to reveal the mask overlay is to hover over an adjustment pin with your mouse. As you can see in the image below, Lightroom will color the areas of the image that the adjustment has been applied to, and it will be darker or lighter depending on how heavy the adjustment is in different areas.
That’s great to review the bounds of an adjustment after the fact, but what about while you’re making the adjustment? Fortunately, Lightroom has thought of everything! With the adjustment brush tool on and an adjustment pin selected, you can press O to toggle whether or not to show the mask overlay for the selected adjustment. You can also toggle this option by displaying your toolbar (T) and clicking the Show Selected Mask Overlay box.
Finally, sometimes red is the absolute wrong color for a situation. In the image above, for example, it’s hard to see if the adjustment bleeds in to the red chair on the left. In that case, you can cycle through the available mask colors (Red, Green, White, Black) by pressing ⇧O or by navigating to Tools > Adjustment Brush Overlay and selecting a color there.
Especially when an adjustment is faint or hard to see, the mask overlay will help you see what areas of an image are affected by an adjustment, ensuring that your local adjustments stay local and are applied exactly where you want them. Now, there will be no more secrets between you and your adjustment brush. May you live happily every after!