Change iMessage Bubble Color on OS13

The blue messages as iMessages, Apple's messaging service. The green ones are SMS messages, the carrier's standard messaging service.

You can't change the colors.

You didn’t show any messages that had different colors. Green is the color of an SMS message.

On my device some of my contacts messages bubbles in iMessages are different colors. Why is that if you said that those colors couldn’t be changed

No, the color of the text bubbles cannot be changed.