int r, g, b = 0;
int x, y = 0;
int prevX, prevY = 0;
void setup() {
       size(255, 255);
       stroke(255);
     }
		  
     void draw() {
       r = mouseX;
       b = mouseY;
       background(r, g, b);
       line(prevX, prevY, x, y);
     }
     
     void mousePressed() {
       prevX = x;
       prevY = y;
       x = mouseX;
       y = mouseY;
       g = (mouseX + mouseY) / 2;
     }