Using a traffic light metaphor, the coloured circles enable you to see if a card has capacity to travel at a glance.
Green says you can travel. Orange warns you of an impending change. Red means you can't travel.
The type of circle displayed depends on what kind of credit you have on your Myki. A hollow circle is shown when your card is charged with Myki Money, and the circle is filled when you have a Myki Pass.
Myki Money --> ○
The card's balance is too low to travel.
The card has $10.00 or less credit.
Your card has a balance of more than $10.00.
Myki Pass --> ●
An extremely rare state where a pass has been applied to a card with a negative Myki Money balance. Even though the pass is there, Myki won't allow you to touch on. 😐
If you're seeing this state, add some Myki Money to your card to enable the Myki Pass.
A card's pass will expire soon.
Pass is active. More than 24 hours remaining. Myki Money is in credit. Everything is fine.