As with most aspects of photoshop, edge control is key.  Knowing how to modify colors is great, but only if you can control where you're applying the change.  After all, it's rare that you'll want to modify your entire image at once.  More likely you'll only want to effect a small region of the image.  Masks are one option, but this video focuses on the rarely discussed "Blend If"

AuthorMatt Kohr