Change Your Message Color
If you want to control the color or font that Outlook uses when you compose or reply to an e-mail you can go to File > Options find the Mail section along the navigation pane on the left and then the fourth button down is "Stationery and Fonts". Click that.
You'll see this dialog box...
From here you can specify what font you want to use (including color) when you create a new message or a reply.
CAUTION: Don't get too carried away
with fonts in e-mail. It may be tempting to use pink Edwardian
Script on a green background but really, don't. (especially in
business e-mail). It's hard for your recipient to read and
frankly kind of annoying for most people.
Special tip - if you want Outlook to automatically use a new color for your text when you're replying or forwarding text, just check the box about 2/3 of the way down the dialog box that enables that.
Notice that you can specify different fonts for replying as opposed to sending a new message - though in most cases you'll probably want to keep your fonts consistent.
Outlook does let you specify fonts for plain text messages but keep in mind that plain text, more or less by definition, doesn't preserve much formatting so there's no point in setting fancy font choices for plain text messages.
It's worth experimenting a bit to see which fonts are most readable and most consistent with your professional image - not just typefaces but sizes too. I currently use Verdana 12 point as my body text and I've found it to be readable but not too blocky.