IMessage blue and green text messages?

Today, my mom was texting on her iPhone 5c or something and she noticed that her messages were green and blue.
For example:
My mom texted one person.
Mom: hey honey (printed in blue. Sent to me)
Mom: have you done the dishes? (Printed in green, and sent to me seconds later)
So like I heard that green means you pay and blue means it's free. But she texted me within a minute and one text was green and the other one was green. Like it switched on the same person and instantly.
I know the picture pretty much explains in but: why does it switch from free messages into text messages that you pay forIMessage blue and green text messages - 1

Sometimes iMessage isn't available so maybe she sent you a message as normal txt and then iMessage became available

It could have to do with data, I also have that. It only uses iMessage when it's available. I also have that with one of my buddies that allow texting and iMessaging. I think it just haves to do if your available for iMessage

Haha

Sometimes iMessage goes unavailable. That happens to me all the time. And blue doesn't necessarily mean free. If you're on Wi-Fi yeah it's free. But if you aren't it switched to data. And if you switched from Wi-Fi to data in the process of sending the message it'll switch from iMessage to text message.