Set the number of screen colours
The first computer I bought (in 1983) plugged into my TV — it didn’t have its own monitor. Soon after that computer screens commonly used black and white, or perhaps orange or green and black. The first Mac I used (1989) had a black and white screen.
This was called 1-bit colour because the computer needed only one bit of memory to display these two colours.
A “bit” is basically a switch which can be either off (black) or on (white). It refers to the video RAM — the RAM (memory) set aside for displaying information on the screen. If you have two switches, or bits, then there are 4 possible positions: both off, both on, one off and one on, one on and one off. With 8 “bits” you have 256 possible combinations, and so on.
My next few Macs gradually added shades of grey — 4 shades of grey, or 16 or 256.
Now, to display colours, the computer needs to blend 3 kinds of light: red, green and blue (RGB). The early machines could display only a few colours: 16 or 256. More recent computers, much more powerful, can display thousands or millions of colours.
You can choose how many colours to display by going to the Control Strip and clicking on the module just to the left of the resolution checkerboard. The colour icon is the one with stripes of colour on it. Try switching around between the different levels of colour.
To best see what difference this makes, try viewing a picture while you do this. You’re likely to see a certain “graininess” at only 256 colours, while at millions of colours it should all be quite smooth.