1. add curves adjustment layer. curves dialog will pop up.

2. double click the highlight eyedropper and the color selector will pop up.

3. click on the target color. click okay on color window. click okay to set as new defaults.

4. click on the area of the image you want to CHANGE the color. the entire image should change to that color.

5. invert the mask by hitting CTRL or CMD "I".

6. paint with a white brush.

This is an excellent video by professional retoucher  Gry Garness retouching an image using many of the tools we've worked with all semester: Smart Objects, PuppetWarp. Transform, Warp, Layer Mask, Clone Stamp and others. It is almost half an hour long but is really worth watching how she uses the tools in her work.

Julia Kuzmenko McKim has a great article and video about what really happens with Frequency Separation at fstoppers.com

"Chances are you have already learned what Frequency Separation (FS) technique is, as it became mainstream in the past few years. However, many FS technique users actually know very little theory behind it, thus have little control over its implementation. I’ve set out to research and collect all the important and useful information about it, so we can together learn how to become better at it."

Michael Woloszynowicz has an advanced tutorial using frequency separation and dodging and burning called "How to Fix Common Hair and Skin Issues with Texture Grafting."