How to change the color of the bubble message?

You are unable to change the color. Green is SMS and blue is iMessage. You can increase the contrast of the color by going to Settings > Accessibility > Display & Text Size > Increase Contrast. This will affect the contrast on everything, not just Messages.

Hello robinsimon,

You cannot change the color of an iMessage bubble.

Blue - Means it was sent securely over iMessage (MMS)

Green - Sent un encrypted (SMS)

What is the difference between iMessage and SMS/MMS? - Apple Support

The closest you can get is by sending "happy chinese new year" and this will temporarily turn the iMessage bubble red and send the celebration screen effect.

Hi! This isn't a feature, but you can suggest it to Apple for future software updates: Product Feedback - Apple. :)

Have a great day!

The color has significance and while you could suggest Apple makes it more flexible, some color difference is likely deemed a useful feature.

Read this document about iMessage (blue) and SMS/MMS (green) text messages --> What is the difference between iMessage and SMS/MMS? - Apple Support

- iMessage = blue bubble. This is message sent between Apple devices only, over WiFi or cellular-data.

- Text Message = green bubble. These are 'regular' text messages (SMS) or a text+media messages (MMS) sent over a cellular text-messaging plan (note it says "Text Message" at the top). These can communicate between Apple mobile devices and non-Apple phones.

Also, at times the Carrier may have issues with traffic or end users'

accounts may affect the color of these message bubbles; so you

could send an iMessage between iPhones, Mac, or iPad; as 'Blue'.

But later that can change to 'Green'; from previous experience

and between Apple product users. So my suspicion remains to

set some blame for this happening ~ on the cellular provider's

own bandwidth and/or the user's account messages backlog.

While this determination is part speculative, facts remain true.

The mystery where these vary, can be subject to new reality.