What Rastan said is exactly right.
When you are merging photos together you can make them look more natural by tweaking the colors. Reds are "warm" colors while blues are "cool" colors. You can reduce the warmth of an image by reducing the reds. The girl on the left has reds reduced. You can do this to any image or any layer of an image by clicking on image / adjustments / curves in photoshop. My example was done in 2 seconds but of course you want to experiment a while with various tweaks to get the image you want of course.
You can also try searching for terms like "color theory", "color matching" etc in Google and probably find some good links. One cool group of items I found is searching for "Sci-fi colors in movies". The best one is "Yellow and Teal in Transformers". Really interesting.
http://theabyssgazes.blogspot.ca/201...ease-stop.html
http://www.miletbaker.com/2011/12/13...-transformers/