fileName  = "python.gif"
red       = False
green     = True
blue      = False
########################

from pygame.image import load, save

pic = load(fileName)

for x in range(pic.get_width()):
    for y in range(pic.get_height()):

        c = list(pic.get_at((x,y))[:3])
        deg = (c[0]*red + c[1]*green + c[2]*blue) / (red + green + blue)
        
        pic.fill( [deg]*3, ((x,y),(1,1)) )

save(pic, "%s_%s%s%s.bmp"%(fileName.split('.')[0],
                           'R'*red,
                           'G'*green,
                           'B'*blue))