Why do some of my messages have a blue background?

iMessage (Blue) -

• iPhone to iPhone: requires internet (Either over WiFi or Cellular Service) as it is sent over internet protocol. Needs Activation on your device.
• iPhone to Android: Not available

SMS/MMS (Green) -

• iPhone to Android: does not require any internet it is sent over cellular voice protocol. Needs to be enabled by your Carrier for the SIM used by you.
• iPhone to iPhone: When iMessage is not activated or the device is not connected to the internet either over cellular or WiFi

It will turn from Blue to Green if you have enabled "Send as SMS" and whenever your iPhone is not connected to the internet it is sent as SMS (green) to all iPhone users.

You cannot change the colors.

Hello vonniefromnrh

Messages with blue background (or blue message bubbles) are messages transmitted over Apple's iMessage service. This means both you and the message recipient are using iMessage.

Messages with green background (or green message bubbles) are messages transmitted over the traditional SMS service. This happens when one party is not using the iMessage service, the message is sent via SMS.

Regards

Zarke

Unfortunately, you are not able to change the background colours of the messages bubbles.