Something to think about:
function setup() {
createCanvas(600, 600);
background(255);
}
function draw() {
background(255);
for(i = 0; i <= 100; i = i + 10){
//fill(3*i, 0, 0);
circle(300, 300, 100-i);
}
}
Try it with and without fill.
It’s all about layers!
:)