i don't think you can change the default color of the tracks...
you can use the preference dialog referring to default track colors, but if you choose the default color coding to be by track type, you can't change the default colors, and you'll have the ugly green
you can choose the default color coding to be something else than track type...or NONE: in this case evry new track you'll create will be black and you can assign any colour you like without seeing that green :)
but this won't help you to avoid changing colors....