When working with masks in Lightroom’s Develop module, such as the Graduated Filter, Radial Filter, or Brush tool, you can easily preview the mask by either using the checkbox in the Toolbar below the image, or with the keyboard shortcut O. By default, this mask is red, but if you are processing a photograph that has a lot of red color in it, the mask may be difficult to see. So thankfully Lightroom has a feature which allows you to change this preview color, and there are a few different ways to do it.
Method 1: Keyboard Shortcut
First, make sure that you have a mask created and visible in your image (use the keyboard shortcut O to view the mask). Then, hold the SHIFT key and press the O key to cycle through the colors (red, green, white, and black).
(Click on the images below to view them larger)
Method 2: The Menu
The other method to changing the color is in the menu. Go to Tools > Adjustment Mask Overlay, and then select the color from the list.