Different colors create different perceptions of depth. Depending on the ground they stand on.
size(500, 500);
background(0);
color light = color(200);
color dark = color(50);
translate(width/2, height/2);
noStroke();
rectMode(CENTER);
fill(light);
int y = 0;
int x = 0;
int w = 100;
int h = 200;
rect(x - w, y, w, h);
fill(dark);
rect(x + w, y, w, h);
saveFrame("out.png");