With the Default Text Styling gadget offered by Google Labs, now you can set the default font, font size, bold, italics, color and background color to be used in your GMail Compose window.
This option is off by default. To enable this lab feature, head over to the ‘General’ tab on the ‘Settings’ page, and change the default text styling to better suit your preferences. You can set whichever color and style fonts as default text while composing or replying to emails.
This is one of the imporant feature missing in GMail. But this feature does not seem to work on the signature. If you have a feedback or suggestions, post it.