When you count cells, you often mix them with a dilute solution of a dye which is excluded by living cells but can cross the membrane of dead cells -- this allows you to count viable and dead cells separately. Probably the most common dye for the purpose (at least, the only one I've ever used) is Trypan blue, which is a very pretty blue color.

Everyone has their own ways of adding the dye; I tend to recycle the lid of a discarded culture dish as a mixing surface, pipetting 10 µl of 2X Trypan blue in buffered salt solution onto the lid and then mixing with 10 µl of cell suspension. Since the cell counter only takes 10 µl, that leaves half of each mixture drying in spots all over the lid as I count my way through my cultures.

One day I decided to take some photomicrographs of the resulting patterns. The crystals are salts; I think the dye tends to dry into blobs rather than crystallizing. The round things that look like alien eggs in images 2 and 3 are what was left behind by air bubbles.