Hello, friend!
The short answer: iMessages are blue, SMS messages are green.
The long answer: iMessage is Apple’s proprietary messaging service that replaces the traditional text messages (SMS) transmitted by your cell phone carrier. iMessage has more security and more options than SMS, but can only be sent between Apple devices (iPhone, iPad, Mac, etc.)
There could be two things happening here. Are your messages showing green for some texts and not others? Or are all texts green?
If only some texts are green, it simply means the person you are messaging has a non-Apple device (and that’s a real shame). Those green messages are being sent as SMS (texts) by your carrier, and there’s no way to change that.
However, if all your messages are green, and you’re certain you’re messaging your friend’s iPhone, you just have iMessage disabled. To enable iMessage, go to Settings, scroll down, and tap Messages:
Then, tap the top button to toggle iMessage on:
You’ll probably need to sign in with your Apple ID. Set that up, and you should now be sending blue bubbles!
Hope this helps!