python - Image processing: Trying to make a photo negative? -

i understand make image negative, have change rgb values current value being subtracted 255.

what wrong following code?

def negative(im): height=len(im) width = len(im[0]) row in range(height):     col in range(width):         red = im[row][col][0] - 255         green = im[row][col][1] - 255         blue = im[row][col][2] - 255         im[row][col]=[red,green,blue] return im 

it returns error "tclerror: can't parse color "#-1d-c-2""

your problem getting negative numbers. think should doing 255 - x rather x - 255