http://www.openprocessing.org/sketch/113285
int x;
int y;

int r;
int g;
int b;

boolean colorchange;

void setup () {
noStroke ();

size (400,400);
background (255);
r=200;
g=255;
b=230;

colorchange= false;

}

void draw () {
x= mouseX;
y= mouseY;

fill (r,g,b);
if (mousePressed==true) {
ellipse (x,y, x/2, y/2);
if (!colorchange) {
r–;
g–;
b–;
}
}
}

I uploaded a Bridget Riley painting from google onto processing

size (600,600);
PImage img;
img= loadImage (“http://nadav.harel.org.il/Bridget_Riley/Loss.gif”);
image (img, 0,0);