Question: Why do you stain bits of people pretty colours?

Paul Laurance-Young answered on 9 Nov 2019:

Body tissues are normally transparent or slightly gray when you cut them into thin sections, so when you look at a section of, say a bit of skin, under a microscope (without any stains) – all you will see is a vague gray outline, maybe some weird fuzzy looking stuff in the middle and then lots of holes where the fat underneath hides.
Using a stain, such as haematoxylin and eosin (which stains the cell nucleus purple and everything else different shades of pink) lets me see all the different structures that make up the skin: The cells that make up your skin proper, the packing material in the middle (full of blood vessels, sweat glands, oil glands and hair) and finally the underlying fatty tissue.
Sometimes I may need to see if a specific type of tissue has been changed (moved out the way or destroyed by an expanding cancer or built up because it is the cancer) so I can use different stains to show these specific tissues: Elastin stain is black and shows the elastic fibres around arteries, Van Gieson stain shows collagen to be bright red, while all other tissue is yellow; PAS stains sugars a funky purple/magenta colour, the Gram stain lets me see bacteria as pink or purple dot. ALcian Blue lets me identify mucus (which stains it an almost electric blue) in normal and cancer cells. There are literally hundreds of different staining combinations – its part science, part what you think will look really great!