Use your country code for the contact you are sending to. I had a similar but different issue a while ago. Any time I started a text to someone who I KNEW had an iPhone it was green, but any replies I made were blue. I asked here and someone else stumbled upon the solution to this.
In my case instead of 555-123-4567 I had to put in +1 for each iMessage contact so it became +1555-123-4567.