Iphone new message contact color?

On a iphone, when you go to create a new text message, and you hit a letter, some show up grey, some blue, some green. What does that mean? And when you hit a letter, and the first number comes up, does that mean you ve recently talk to them?

Grey would mean that number is not valid to text to/hasn't been texted
Green would mean that they can be/have been texted
Blue would mean that they can be iMessaged (not text message)