[Python] How to decrease the brightness of an image in PyGame?

I know there's a similar question here - How can I change the brightness of an image in pygame? - but the answer only explains how to increase the brightness, without referring to how to decrease the brightness. I want to create an animation where an image increases in brightness, but then decreases in the same manner. I've tried using this code:

running = True
brightness = 1
brightstep = 1
while running:
colour = (brightness, brightness, brightness)
canvas.fill((255, 255, 255))
canvas.blit(bg, (0, 0))
canvas.blit(img, (0, 0))
if brightness == 51:
brightstep = -1
brightness = 50
elif brightstep == 1:
brightness += brightstep
img.fill(colour, special_flags=pygame.BLEND_RGB_ADD)
elif brightstep == -1:
brightness += brightstep
img.fill(colour, special_flags=pygame.BLEND_RGB_SUB)
for event in pygame.event.get():
if event.type == pygame.QUIT:
running = False
pygame.display.update()


The images have already been loaded so that's not causing the error. I know it's something to do with the special_flags when I'm decreasing the brightness.

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