What does it mean if your texting a person with another iPhone and the messages when u send their blue now their turning green and and when I call his phoneIt goes straight to voicemail even when I try calling from a private number as well

Their phone may be on Do Not Disturb. Their phone may be off. They may not have a data connected (either WiFi or cellular). They may be ignoring you. Or some combination of the above.

It just means that they have swapped from using iMessage to normal Text message, therefore this does not run off your internet anymore like your iMessages do, it comes out of how many texts you get a month etc.