On iOS 7, the Messaging tab displays a conversation with a green background or a blue background. If you have used iOS 6, the message has been sent to the green or blue send button. Are these colors random and your message is sent as a regular SMS or email?
Green – for SMS text message
The Green Background or Send button indicates that the message will be sent as a regular SMS text message using the message plan from your mobile phone provider. Regularly pressed rates are applied based on your text plan.
An SMS text message will automatically be used to send text to a non-Apple device. For example, if you send a text message to your main cellphone (smartphone) or smartphone other than your iPhone (Android, BlackBerry, Windows …), iPhone will display your message in green background (or click the Green Send button for iOS 6 users) . SMS messages have been the main cell phones for years, even before the iPhone was discovered.
For Blue iMessage
A blue background or send button indicates that the message will be sent in iMessage format.
IMessage was sent by Apple instead of your mobile phone provider. The IMessage is sent over the Internet, not through the mobile phone controller. There is no charge for text messaging from your mobile service provider when sending iMessage.
When an email message arrives with an Apple device, your iPhone will know if you have an electronic identifier and send the message as iMessage. On the iPhone, the message is displayed in blue (or click the blue send button for iOS 6 users).
Apple users direct Apple ID-based iMessages. That’s why anyone with an iPad or Mac computer can send iMessages. However, normal SMS messages can not be sent because the iPad or Mac does not have access to the mobile phone channel.
Whether the message has been sent as a regular SMS message, or the iMessage will automatically detect the iPhone. There is no need to worry about choosing one or choosing another. However, if you want some control when sending SMS or iMessage, check out the options for the iPhone settings.