[mewse]

Green bubble is messages you sent via SMS (and so may have been charged by your carrier depending on your cellular plan)

Blue bubble is messages you sent via iMessage.

All incoming messages are grey, regardless of whether they were sent to you via SMS or iMessage.

[beardyw]

> and so may have been charged by your carrier depending on your cellular plan

I don't know of any UK plan that charges.

[dkdbejwi383]

Must be a couple of decades since I was last charged for SMS

[bdavbdav]

Try sending an image on green.

[dfawcus]

PAYG plans do.