// The colors in a rainbow 
color[] palette = {color(255, 0, 0), color(255, 127, 0), color(255, 255, 0),
                   color(0, 255, 0), color(0, 0, 255),   color(111, 0, 255),
                   color(143, 0, 255)}; 

size(600,400);
int numColors = palette.length;      // 'length' field automatically defined for arrays
float bandWidth = 10;
strokeWeight(bandWidth);
noFill();

for (int j=0; j < numColors; j++) {
  float radius = 0.5*width - bandWidth*(1+j);
  stroke(palette[j]);      // pick appropriate color
  arc(0.5*width, height, 2*radius, 2*radius, PI, TWO_PI);
}