green bubbles vs blue bubbles to same contact

No that’s not what it means. That person obviously has an iPhone or an iOS device that they have registered for iMessage. iMessages are the blue bubbles. Even though iMessage works over WiFi, sometimes those messages will go through as SMS (green bubbles) if the WiFi connection is flakey or bad. SMS a messages are are a function of your wireless carrier, iMessages are sent over WiFi or cellular data through the Apple App.

About iMessage and SMS/MMS

Learn why some of your message bubbles are blue or green.

You can use the Messages app on your iPhone, iPad, or iPod touch to send messages. Those messages are sent as iMessage or SMS/MMS. Learn more about the difference between the message types.

https://support.apple.com/en-us/HT207006

Also check to see if the phone your sending to is registered for iMessage