(If you change from a 256-colour mode to a 32-thousand colour mode on an Acorn computer, the colours are sometimes not correct. I don't know why. Close !Java and click on the reload icon on your browser toolbar.)
You will see that the squares are arranged as 36 groups of six squares, with the six squares in each group being presented diagonnally. The groups show the effect of increasing the blue component (diagonally) for each of the combinations of the red component (horizontally) and the green component (vertically).
On web pages you are recommended to use only colours which are in the 216 colour cube because you can be reasonably certain that these will give no technical problems on any computer platform, although the colours may not be precisely the same. (The colours will be very different if someone is using only a black and white monitor or a mode capable of displaying only 16 colours, of course.)
If your browser is Java-enabled, on the right of the 216 squares you will see the numerical values of the red, green and blue components of any square over which the pointer rests, both as decimal numbers and as hexadecimal numbers, together with a copy of the square.
The numbers are in steps of 51 (33 in hexadecimal) and range from 0 to 255 (0 to FF in hexadecimal). Thus, a colour component may have a value of 0, 51, 102, 153, 204, or 255 (0, 33, 66, 99, CC or FF in hexadecimal).
The background colour of this page was originally the same as the pale blue square which is on the very bottom row, second from the right, and has a colour of #CCFFFF where the CC represents the red component, the following FF represents the green component and the final FF represents the blue component.
It now has a colour of #00FFFF which is on the left of the very bottom row.