Just to send texts in iMessage it is a minuscule amount. MMS through IMessage will use a little more. If your on unlimited data then obviously nothing to worry about. But even if not its a small amount over your data plan. There are some other threads that go into detail on this subject if you use the search feature on this site. When on iMessage it does not count against texts messages because it uses data over Apples servers.
Yep if it's on and available it changes to blue. If its off or unavailable it will be the normal green. It will only be an iMessage if it owes to another device with iMessage turned on. It will switch back and forth automatically.