The createImage() function provides a fresh buffer of pixels to play with. This example creates an image gradient.
Original Processing.org Example: CreateImage
// All Examples Written by Casey Reas and Ben Fry // unless otherwise stated. PImage img; void setup() { size(200, 200); img = createImage(120, 120, RGB); for(int i=0; i < img.pixels.length; i++) { img.pixels[i] = color(0, 90, 102, i%img.width * 2); } } void draw() { background(204); image(img, 33, 33); image(img, mouseX-60, mouseY-60); }