How can I change the default text color in Mail?
Unfortunately I'm unaware of any way to do this without a workaround. Here it is:


Go to preferences -> signatures and create one that uses the font color you want to use. You have to select the text, right-click, go under font and pick the color from there (at least on my Lion machine)
Set that signature as your default
When you compose a new message, put your cursor on the beginning of the line that the signature is on, hit enter a few times, and when you start typing your message it will be in that font/color. There will be a blank line at the top that is formatted black, which you can backspace to delete, just make sure you've started typing something first or it will set it back to black.