Unable to iMessage Between iPhones

Question

I have an iPhone 4S.  My wife has an iPhone 5.  When I try to send her an iMessage, the background on the message is green, not light blue - in other words, we are texting, not iMessaging.  iMessaging has been enabled on both phones.  Both phones are connected to 3G at the time.  Why won't iMessageing work between the two phones?  I am generally (but not always) able to iMessage with other iPhones.  Why between my wife's and my iPhones?
I did notice this - when I go into Settings/Messages/Send&Receive, in the field under "You can be reached by iMessage at:", I see my iPhone number in there, ut it is grayed out.  I don't seem to be able to activate it, i.e., I don't know how to "un-gray" it.  Might that have something to do with the problem?

Answer 1

OK - here's what my Send/Receive looks like now:
Apple ID (My Apple ID is correctly stated here)
Under "You can be reqched by iMessage at," I have . . .
My cell phone number (checked and grayed out)
My wife's cell phone number (checked and active - not grayed out)
My wife's email address (checked and active - not grayed out)
My email address (checked and active - not grayed out)
Still in this section, it then says: "Add Another Email"
The next section is: "Start new conversations from:"  Under that is . . .
My cell phone number (not checked and active - not grayed out)
My wife's cell phone number (checked and active - not grayed out)
My wife's email address (not checked and active - not grayed out)
My email address (not checked and active - not grayed out)
It gives me the option of deleting the two email addresses in the "You can be reached by iMessage at" section.  I am unable to do anything with any of the items listed in the "Start new conversations from" section.
What should I do and, more importantly, how should I do it?  I'm afraid I am going to need a step-by-step on this if you can - I simply cannot make this sucker work.