Why would a text on my iPhone turn from blue to green?

But what if you've been texting them and then all of a sudden your text sends a minute later and turns green?

Do they still get the message if bubble turn blue to green?

Blue messages are iMessage where as a green one is an ordinary text message.iMessage's get sent over the Internet & are free so your Internet or wifi connection must of dropped out so it got sent as a regular text message