Gradient.py

import pygame, sys
from pygame.locals import *

def to_color(a, a_max, b, b_max):
    return (255,
            int(float(a)/a_max*255),
            int(float(b)/b_max*255))

def pixel(x, y, size):
    return (x * size, y * size, size, size)

def draw_gradient((width, height), display, box_size):
    m_max = width / box_size
    n_max = height / box_size
    for m in range(m_max):
        for n in range(n_max):
            pygame.draw.rect(display,
                to_color(m, m_max, n, n_max),
                pixel(m, n, box_size))

pygame.init()
width = 500
height = 400
display = pygame.display.set_mode((width, height))

draw_gradient((width, height), display, 10)

while True:
    for event in pygame.event.get():
        if event.type == QUIT:
            pygame.quit()
            sys.exit()
    pygame.display.update()