Texts colors

It will be blue if you are communicating with those who an iPhone and have iMessages activated. It will be Green if you are communicating with those who are either non-iPhone users or iPhone users who do not have iMessages setup. Green messages are sent through SMS and Blue messages are sent through Apple's servers.

Axel F.

iMessages are in blue and SMS/MMS messages are n green. For more info, see:

About iMessage and SMS/MMS - Apple Support

iMessages are texts, photos, or videos that you send to another iPhone, iPad, iPod touch, or Mac over Wi-Fi or cellular-data networks. These messages are always encrypted and appear in blue text bubbles

If you aren’t using iMessage, you can use SMS/MMS. These messages are texts and photos that you send to other cell phones or another iPhone, iPad, or iPod touch. SMS/MMS messages aren't encrypted and appear in green text bubbles on your device.

The Blue text bubble is for iMessage, that is used between iOS devices. The green text bubble is for SMS, which are non-iOS devices, such as Android phones.