How would this work? My understanding is that there’s no colour in peripheral vision. I guess a change in shade/intensity would be enough?
There is minimal color, yes, but I suppose one's brain will learn to "fake it" once it figures out size-color correlation during those times you're paying it direct attention.
You are totally right. I am going to implent that feature, after using it for a while I realised that depending on the background it was hard to tell if the ball was growing or shrinking
Just to add to this idea, it's important to understand when the circle is near the start or end especially. I found myself not sure when to start breathing and not sure when to stop. I think the color idea will definitely help but sure if this specifically addresses this issue.