Why are my messages to someone green when they used to be blue?

Possible that that "someone" is not using an iPhone?

If they are using an iPhone, they may have turned off the option to use iMessage. A poor connection can also go as a regular message.

Swildish wrote:

Why are my messages to someone green when they used to be blue?

Either you or the recipient is in an area with a weak wifi or cellular data connection or has imessage turned off.