From this paper:
http://rsif.royalsocietypublishing.org/content/early/2012/09/22/rsif.2012.06... The following quote (emphasis mine): Humans possess three cone visual pigments for conveying colour information that is said to allow humans to be able to detect approximately 10 million unique colours [8,9] but only distinguish about 30 shades of grey [10]. (Let's ignore recent movies of dubious merit in this discussion. =P) In the new MPL imshow plugin (which I recently wrote), we switch from grayscale to cubehelix when the dynamic range is too low to be displayed on a common monitor (1/255): https://github.com/scikit-image/scikit-image/blob/master/skimage/io/_plugins... Maybe the threshold should be when the difference is imperceptible to most humans (1/30)? Juan.