put this in front of the ellipse function in the "storing input" example
fill(color((mouseX/width)*255));
that gives you black on the left side, and white on the right side..

To explain:
mouseX is a value between 0 and width, (unless it is outside, but then you'll have to click and drag)
dividing mouseX by width, gives you a decimal value between 0 and 1, multiplying this with 255, (which is the max value for the color function) gives you a value between 0 and 255, which is black and white..

good luck

Try playing with other numbers, and minus, plus, multiply etc to see something change.. should give you some ideas on what should be changed, and what shouldn't.

Also.. colorMode is for setting RGB, or HSB, which is different ways to use a color.

try by modifying this...



void setup()
{
 size(200, 200);
 background(0);
 smooth();
 colorMode(RGB,255,255,255,100);
}

void draw()
{
 // Call the variableEllipse() method and send it the
 // parameters for the current mouse position
 // and the previous mouse position
 variableEllipse(mouseX, mouseY, pmouseX, pmouseY);
}


// The simple method variableEllipse() was created specifically
// for this program. It calculates the speed of the mouse
// and draws a small ellipse if the mouse is moving slowly
// and draws a large ellipse if the mouse is moving quickly

void variableEllipse(int x, int y, int px, int py)
{
 float speed = abs(x-px) + abs(y-py);
 stroke(speed);
 fill(speed*15,speed*3,255,100);
 ellipse(x, y, speed, speed);
}