iMessage acting strange iPhone

When I message a specific contact using messages, any response to my message opens in a new window/message and the color of the message is blue instead of green.  This contact is another iphone user and I have other iphone users I message with and do not have the same problems.  Is this a known problem?  Do I need to delete my one contact and re-enter their details?  Any idea why the new message window is blue versus green?  We have the same provider, but that should not be the issue either.

Any assistance you can provide is greatly appreciated.

This was happening before the update to OS 5.1 and has happened since as well