Rainbow

Press Play.

import turtle, time
screen = turtle.Screen()

#global red, green, blue
red = 256; green = 0; blue = 0

def changeColor(addSub, rgb, var):
    global red, green, blue
    for i in range(256):
        if addSub == "+":
            rgb+=1
        else:
            rgb-=1
        if var == 'red': red = rgb
        elif var == 'green': green = rgb
        else: blue = rgb
        screen.bgcolor(red/256, green/256, blue/256)
        time.sleep(0.01)

while True:
    changeColor("+", green, 'green')
    changeColor('-', red, 'red')
    changeColor("+", blue, 'blue')
    changeColor('-', green, 'green')
    changeColor("+", red, 'red')
    changeColor('-', blue, 'blue')