Python code here.

             
def setup():
    createCanvas(600,600)
    noStroke()
    rectMode(CENTER)


def draw():
    colorMode(HSB,100)
    h = map(mouseY,0,600,0,100)
    background(h,100,100)
    fill(100-h,100,100)
    rect(300,300,mouseX+1,mouseX+1)