Click on the eyedropper tool to select one of 70 colors: click on the swatch you like to change default text color, keeping in mind that it should offer a high enough contrast to be easily legible to recipients, especially if you use a stationery (i.e., non-white message background). Tip: when composing messages, use the formatting toolbar to override default fonts/colors.
Tip # 2: if you have setup an email signature, consider making it match your new defaults.