1) The left side of the page controls the background color and the right side controls font (or text) color.
This Color Display Program shows 216 colors. These are the so-called "safe colors" (meaning colors that ALL browsers will display correctly).
If you want to experiment with over 16 million colors, try this link:

2) Each of the 3 rows consists of the characters:

Rolling the mouse over any of these characters will change the colors.
Row 1 controls the amount of RED,
Row 2 controls GREEN and
Row 3 controls BLUE.

3) With EVERY color change, the 2 output boxes display the "hex value" of that color.
For example, the value for blue is "00 00 FF".

4) This program is quite handy for experimenting. When you create a color you like, write the numbers down and use it on your webpage(s).

But enough talk - you'll learn quickly just by doing.
Have fun finding new colors and color combinations.