Good question! I’m currently growing cancer cells on my materials because they grow really easily and are cheap making them very good for testing whether a material that I make might be good for growing heart cells on – which are much more difficult to grow and very expensive! These cancer cells are definitely white when I look at them under a microscope.
For other cells it depends on their function. Our skin cells contain a pigment called melanin that protects us from the UV radiation from the sun, this gives our skin it’s colour. Red blood cells are red because they contain iron, which is required for the cells to transport oxygen round our body. Our muscle can be red or white depending on its function, which affects the amount of iron present in them (white muscle having much less)
Octopuses have blue blood cells because they use copper to carry the oxygen round their body rather than iron!
Most cells are colourless but, as Marcus says, if they have other molecules in that give them some colour. When lots of cells are together they can look colourful, but when you only look at a few they look transparent. It’s a bit like looking at the sea being blue but if you were to scoop some seawater up in a glass it would look clear.
This is really obvious in people who suffer from Albinism where they have a genetic disorder which means there body cannot produce melanin which is the pigment in our skin, eye and hair with give it colour.
When we use cells in the lab and look at the individually, we often have to use fluorescent dyes of green or red in order to see/count them on the black background.