If I may chime in on Rod’s suggestion…the Quickmask is easily accessible by simply pressing the “Q” keyboard key. And by having your foreground and background colors set to the default Black & White which is easily attainable by simply pressing the “D” keyboard key one can alternate between adding to the quickmask selection or subtracting from it by pressing the “X” keyboard key to alternate between the foreground and background colors. Just a quick tip.

Also, you can select the RGB channels which offer different degrees of contrast (the red channel often having the most contrast) to select portions of your image. And these can be further refined by using different filters such as High Pass or from Image Adjustments Threshold to increase contrast.