In iOS 7, the Messages app will display conversations with a green or blue background. If you’ve used iOS 6, you may have noticed that the messages had a green or blue Send button. These colors are not random and actually tell you if the message will be sent as a normal SMS text message or as an iMessage.
Green is for an SMS text message
The green background or the Send button indicates that the message will be sent as a normal SMS text using the messaging plan provided by your cell phone provider. Regular texting rates will apply, based on your texting plan.
SMS text message is used automatically when sending a text message to someone without an Apple device. For example, if you are texting someone with a basic cell phone (other than a smartphone) or any smartphone other than an iPhone (Android, BlackBerry, Windows …), your iPhone will display the message on a green background (or have a green Submit button for iOS 6 users). SMS text messaging has been around for years, even before the iPhone existed and everyone had basic cell phones.
Blue is for iMessage
The blue background or the send button indicates that the message will be sent as an iMessage.
Apple sends an iMessage instead of your cell phone provider. The iMessage is sent through an Internet connection rather than through the cell phone control channel. There are no text messaging charges from your cell phone provider when sending an iMessage.
When sending a message to someone with an Apple device, your iPhone automatically recognizes if the sender has an Apple ID and will send the message as an iMessage. On your iPhone, the message will display with a blue background (or it will have a blue Send button for iOS 6 users).
Apple routes iMessages based on a user’s Apple ID. Therefore, anyone with an iPad or Mac can send iMessages. However, because the iPad or Mac computer does not have access to a cell phone control channel, it cannot send regular SMS text messages.
Your iPhone automatically determines whether a message is sent as a normal SMS text message or as an iMessage. There is no need to worry about selecting one method or the other. However, if you want to have some control over when an SMS or iMessage text message is sent, check out the message options in the iPhone settings.