Let us start with a quick comparison between digital photos (or pictures) and medical images:
Pixel values are less important on photos and pictures. In fact the very same photo may be represented using completely different pixel values e.g. RGB and HSL. For medical images, however, I don't care whether a pixel shows black or white, red, green or blue, purple or teal — so long as there is a colour-bar showing me the pixel values corresponding to the colour scale. The meaning of medical images lies in the pixel value, not the colour. For example:
While gray-scales may invite fond nostalgia of black-and-white televisions, they should never be under-estimated. This colour-map is still widely recognised as the one offering the best medical diagnostic value. Note that gray-scales are not black-and-white. We can have thousands of shades of gray. Black-and-white is limited to just two colours: black and white.
Although one might feel conscious and tries to remove wrinkles and a pimple or two from photos, local features are far more important on medical images than photos. A local feature makes a big difference to a person's life, casting the diagnosis into one of the four types: false negative, true negative, false positive or true positive. The latter pair may open up a whole episode of medical interventions.