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/matplotlib_plugin.py#L51
Maybe the threshold should be when the difference is imperceptible to most humans (1/30)?
Juan.