Taken from http://selfhtml.teamone.de/helferlein/farben.htm

Foreground color:

Enter on the left values between 0 and 255. As an alternative you can also click on one of the colors from the table below.

red:
green:
blue:

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Background color:

Enter on the left values between 0 and 255. As an alternative you can also click on one of the colors from the table below.

red:
green:
blue:

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Click the button to show the color in the frame on the right and to calculate the HEX values below

Resulting HEX values:

Foreground: Background: