Contact colors when typing in a person's name to send a text message

When I am sending a text message and I start typing the person's names I am sending the message too, it brings up several names that start with the same letter. Why are some of the names blue, green, and some gray? What do the colors mean? I am using an Iphone 5 that is updated to the current software.

Example: If I am sending a message to someone named John, when typing the letter 'J' to choose his name when sending a text, it will bring up a list of all the names starting with a letter 'J'. Some are blue, some green, and some gray. What do these colors mean?