Thats called bruising. They usually turn a dark purple/blueish colour when pressed on the cap. Another way to find the species is to take a piece of tinfoil, take the mushroom cap and place it on the tinfoil. In about an hour come back, carefully pick the mushy cap off the tinfoil, and look closely at the color it left behind on the foil. Thats the spores from the cap. We used that as a final guideline to find out the species.
If you want to know more about this, go to
www.erowid.org for specific spore colors. Be careful if you decide to go pick these things in spring, they have "imitation" mushrooms that are deadly.