There are two ways of doing this.
First let's say you have the RGB value 0, 64, 128, and want to add it to the code... do this:
Just insert the RGB value where the HEX value would normally go with this formatting:
color: rgb(0, 64, 128);
Or if you would like to change it to HEX format from within CoffeeCup HTML editor do this:
1. Click on the color-wheel icon in the toolbar
2. Click the "Define Custom Colors >>" button.
3. Then, simply enter the RGB value in the Red, Green, Blue boxes (Red: 0, Green: 64, Blue: 128)
4. Press "OK" and the Hex code #004080 will be generated for you.
Let me know if you need any more help with this.