The whizzes at Bostongraphy have taken an ingenious stab at mapping the autumnal changes in the area's street trees. Using tree inventories taken by local governments (seriously—your tax dollars at work), they were able to approximate where you can take the best gander at the orange, red, yellows and purples of October and early November.
Then head to the outer-neighborhoods of Boston like Hyde Park and West Roxbury.
Leaves of Norway Maples, which overall are the most common of any tree species in Boston (the species is considered invasive, in fact), generally turn yellow in the autumn while those of Red Maples (as the name suggests) are liable to turn red, hence the map colors. Norway maples make up a larger percentage of trees in the outer neighborhoods than elsewhere, while red maples are most prevalent in Cambridge and, for some reason, especially the Fenway-Kenmore neighborhood. So head to the Fenway if you want to see red, I guess.
As Bostonography points out, though, if you're a real leaf freak, then go beyond the sidewalks and side streets of the Hub. Hit the country and gape in suitable awe.
Autumn Streets [Bostonography]