Why is the "Connect" button green/blue/grey?

The "Connect" button is green during the lesson. If you scheduled your lesson for the period from 10:00 to 10:45, the button is green during this time period both in the teacher and the student account.

15 minutes before the lesson the button is blue and a countdown to the start of the lesson is active.

The button is grey for lessons that have been scheduled for later that day or for the following days. The teacher can click even on a grey button, start the lesson and thus confirm, that everything has been set right.