After you had clicked the "Post Reply" icon, it will bring you to a page that is full of options. Just simply look for the top, there's a drop down bar, named [Color], and you will see many colors. Look for the color you want and click it. A pop-up will appear, you type in the text you want in the bar provided, and press the ENTER key on your keyboard.
The text will then be colored by the color you chosen. Understand now?