You do not generally have right of way in a marked crosswalk if you have a signal. Anywhere that doesn't have a signal you have the right of way at all times within a marked crosswalk only, regardless of the color of light, outside of marked crosswalks pedestrians are not guaranteed right of way in MA.

I used to be a Bostonian and am a current MA resident, though I work in Cambridge. I've played more chicken on Comm Ave than I care to admit. :)