We expect light to come in the front of the camera. Shooting with the lens cap on doesn't tend to work so well. But we don't always expect it to come in the back. Here's a real-world example of light leak through the viewfinder.
Nearly 1 in 10 people who look at your photos are likely to perceive colors differently because of some form of colorblindness. Here's a way to use Photoshop to simulate how your photos will look to someone who is colorblind.