Why does it change between green and blue in a iMessages to the same person?

Yes. Green messages are SMS, Blue messages are iMessage.

iMessages can only be sent when both parties have an Apple Device, have enabled iMessage, and have internet access. If either party loses internet access, then the Messages App will default to text messages (carrier based SMS) to try send the message.

Your recipient may be in an area without internet access, either cellular or wifi, but can still recieve SMS messages through their carrier.

Read here for more:

About iMessage and SMS/MMS - Apple Support

Because sometimes it sends the message using iMessage (blue) and sometimes it sends it as an SMS message (green).

It depends on whether they have internet access.

There can be many reasons for this. The area may be a factor if it does not have good cell coverage to get internet but that's not the sole reason. Even if the area does have good cell coverage but the recipient has chosen to not let the Messages App use cell data when away from Wifi then only SMS messages (green) can be sent / received when not on wifi.

The important part is internet access.

The color differentiation shows whether they are receiving the texts as MMS or SMS. In your case, there must times when you're not getting MMS messages.

In order for a message to be sent as an iMessage (blue) both the sender and the receiver must be logged into iMessage and must have an active data connection (either WiFi or cellular). So, if one person is somewhere that they don't get cellular data and aren't connected to WiFi, the message will be sent as an SMS (green) message. This assumes that Settings>Messages>Send as SMS is turned on.