size(530, 200);
noLoop();
background(0);

final short margin = 15, w = 20, gradient = 4;

for (int gray = 0400, pos = margin; pos < width - margin*2; pos += w) {
  fill(gray -= gradient);
  rect(pos, margin, w, height - margin*2);
}

               

                
size(650, 200);
                
noLoop();
                
background(#FF0000);
                


                
final short MARGIN = 15, WIDE = 20, HUES = 0400;
                


                
final int condition = width - MARGIN*2;
                
final int high = height - MARGIN*2;
                


                
final int rects = (condition + MARGIN) / WIDE;
                
println(rects);
                


                
final int gradient = HUES/rects;
                
println(gradient);
                


                
for (int grey = HUES, pos = MARGIN; pos < condition; pos += WIDE) {
                
 fill(grey -= gradient);
                
 rect(pos, MARGIN, WIDE, high);
                
 print(grey + " - ");
                
}
               

2. void setup() {
3.   size(766, 200);
4. }
5. 
   
6. void draw() {
7.   for (int i = 0; i < 255; i++) {
8.     fill(i);
9.     rect(i*3, 10, 3, 180);
10.     noStroke();
11.   }
12. }