There are three main types of color blocks:
This is a basic color block. It has a small square shape and has a color in the middle that represents the color stored internally in this block.
If you click on the color in the middle, a pop-up appears on the screen with a table of 70 colors that you can choose from. Clicking on a new color will change the current color of your basic color block.
Each basic color block that you drag from the Colors drawer to the Blocks Editor screen will display a table with the same colors when clicked.
Internally, App Inventor stores each color as a single number. When you use make color and take in a list as an argument, internally this list is then converted using App Inventor's color scheme and stored as a number. If you knew the numbers for the colors, you could even specify what color you wanted something to be by just setting its Color property to a specific number. If you want to see a chart of colors to numbers, check out this page.