Instead of hitting "fast reply", hit "reply". Then look just above the text entry box. You'll see some buttons like:
B I U Font Size Color
http :// IMG @ Quote Code List
Just click on the Color button, select a Color from its list, and look at the code it pastes into your text.
After the COLOR= tag appears, type something. It will be the selected colour once your entry is posted. Be sure to click "Close all tags" afterwards.
Say I wanted to type this sentence: "The big dog barked." and I wanted "dog" to be red, but the rest to be black. I'd do it like this:
(1) type "The big "
(2) click Color, select Red
(3) type "dog"
(4) click Close all Tags
(5) type " barked."
The result is this:
The big dog barked.