Changing colors

Greetings,
I've already got colors picked out for illustrations. However, in certain settings (storms, etc...) I realize that colors are naturally perceived as being darker or lighter. Is there anyway I can use photoshop to change all colors on an image to a darker shade, so I'm not updating the colors individually?

You could start out trying Levels (CTRL-L) or Curves (CTRL-M) to do global adjustments.