Transparent Colours

size(500,500);
fill(255,0,0,0);
rect(0,0,100,100);
fill(255,0,0,50);
rect(25,25, 100,100);
fill(255,0,0,100);
rect(50,50,100,100);
fill(255,0,0,150);
rect(75,75,100,100);
fill(255,0,0,200);
rect(100,100,100,100);
fill(255,0,0,255);
rect(125,125,100,100);