Don't worry hun', it's simple once ya know how! ;)
If you look up at the buttons above the reply box you see things like 'Bold', 'Italic' 'Underline' etc. Well, go along that top row and you see a drop down menu that says 'Color'. Highlight the text you wnat to colour, and click the menu and you get a list of colors. (Got a cap to show ya, don't worry! Click to make bigger))

Alternativly, you could put the colour tags around the block of text you want to highlight, for example: