Highlight the letter you want to color and then in the tool bar at the top click the drop-down arrow to the right of the large A with the black bar beneath it. Then choose the color you want from the palette that displays. It will look really terrible in your message...something like the following surrounded by square brackets instead of parentheses..... (COLOR="Blue")abc(/COLOR).
When you preview the post it will show the colored letters. You can also do the same with bold, underline, and italics, or different font and size. The markup tags will show in the message editor but won't display when you preview.
Hope that helps...