Imessage phone e-mail conflict

Hello there,

I have a strange question. From my Iphone5(7.0.4) i can send messages via imessage using e-mail. For one of my contacts the "send" button is blue while sending sms to his mobile number(not to e-mail). For another contact, the button is green and it sends normal sms, via e-mail only i can send message via imessage. The issue in summerry, i do prefer to be able to send imessage via the contacts mobile number. How is this possible?