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This example has been modified from The Mobile technology group example : rectangle.py

# random rectangle
 
from appuifw import *
import e32
from random import randrange
 
running = 1
def quit():
	global running
	running = 0
app.exit_key_handler = quit
 
app.screen = 'large'
app.body = canvas = Canvas()
res_x, res_y = canvas.size
 
while running:
	x1 = randrange(res_x)
	x2 = randrange(x1, res_x)
	y1 = randrange(res_y)
	y2 = randrange(y1, res_y)
	color = randrange(0xffffff)
	canvas.rectangle((x1, y1, x2, y2), fill=color)
	e32.ao_yield()

Here's a variation : It bounces a rectangle round the screen leaving a trail in different colors.

from appuifw import *
import e32
from random import randrange
 
running = 1
def quit():
	global running
	running = 0
app.exit_key_handler = quit
 
app.screen = 'large'
app.body = canvas = Canvas()
res_x, res_y = canvas.size
 
dy = 1
dx = 1
x1 = 10
y1 = 10
 
while running:
	x1 = x1 + dx
	y1 = y1 + dy
	x2 = x1 + 10
	y2 = y1 + 10
	if (x1 < 1):
		dx = -1 * dx
	if (y1 < 1):
		dy = -1 * dy
	if (x1 > res_x - 15):
		dx = -1 * dx
	if (y1 > res_y - 15):
		dy = -1 * dy
	color = randrange(0xffffff)
	canvas.rectangle((x1, y1, x2, y2), fill=color)
	e32.ao_yield()

The following does not leave a trailing image.

import e32,random
import time
import appuifw
import graphics
import math
running=1
def quit():
    global running
    running=0
appuifw.app.exit_key_handler=quit
appuifw.app.screen='large'
can=appuifw.Canvas()
appuifw.app.body=can
j,k=can.size
dx=1
dy=1
x1=10
y1=10
while running:
    x1=x1+dx
    y1=y1+dy
    x2=x1+10
    y2=y1+10
    if x1<1:
        dx=dx*-1
    if y1<1:
        dy=dy*-1
    if x1>j-15:
        dx=dx*-1
    if y1>k-15:
        dy=dy*-1
    c=random.randrange(0xffffff)
    can.rectangle((x1,y1,x2,y2),outline=None,fill=c)
    e32.ao_sleep(.01)
    can.clear(0xffffff)
    e32.ao_yield()