This example is for Processing 3+. If you have a previous version, use the examples included with your software. If you see any errors or have suggestions, please let us know.

Button.

Click on one of the colored squares in the center of the image to change the color of the background.

 
int rectX, rectY;      // Position of square button
int circleX, circleY;  // Position of circle button
int rectSize = 90;     // Diameter of rect
int circleSize = 93;   // Diameter of circle
color rectColor, circleColor, baseColor;
color rectHighlight, circleHighlight;
color currentColor;
boolean rectOver = false;
boolean circleOver = false;

void setup() {
  size(640, 360);
  rectColor = color(0);
  rectHighlight = color(51);
  circleColor = color(255);
  circleHighlight = color(204);
  baseColor = color(102);
  currentColor = baseColor;
  circleX = width/2+circleSize/2+10;
  circleY = height/2;
  rectX = width/2-rectSize-10;
  rectY = height/2-rectSize/2;
  ellipseMode(CENTER);
}

void draw() {
  update(mouseX, mouseY);
  background(currentColor);
  
  if (rectOver) {
    fill(rectHighlight);
  } else {
    fill(rectColor);
  }
  stroke(255);
  rect(rectX, rectY, rectSize, rectSize);
  
  if (circleOver) {
    fill(circleHighlight);
  } else {
    fill(circleColor);
  }
  stroke(0);
  ellipse(circleX, circleY, circleSize, circleSize);
}

void update(int x, int y) {
  if ( overCircle(circleX, circleY, circleSize) ) {
    circleOver = true;
    rectOver = false;
  } else if ( overRect(rectX, rectY, rectSize, rectSize) ) {
    rectOver = true;
    circleOver = false;
  } else {
    circleOver = rectOver = false;
  }
}

void mousePressed() {
  if (circleOver) {
    currentColor = circleColor;
  }
  if (rectOver) {
    currentColor = rectColor;
  }
}

boolean overRect(int x, int y, int width, int height)  {
  if (mouseX >= x && mouseX <= x+width && 
      mouseY >= y && mouseY <= y+height) {
    return true;
  } else {
    return false;
  }
}

boolean overCircle(int x, int y, int diameter) {
  float disX = x - mouseX;
  float disY = y - mouseY;
  if (sqrt(sq(disX) + sq(disY)) < diameter/2 ) {
    return true;
  } else {
    return false;
  }
}