Source code in the processing programming language

fullScreen();
noStroke();
color[] cs = {
  color(0),         // black
  color(255,0,0),   // red
  color(0,255,0),   // green
  color(255,0,255), // magenta
  color(0,255,255), // cyan
  color(255,255,0), // yellow
  color(255)        // white
};
for(int i=0; i<7; i++) {
  fill(cs[i]);
  rect(i*width/8,0,width/8,height);
}