This fruit is green, not yellow, with a streaky spot of a faint dusky rose blush and faint, small lenticels, some of which are light and some russeted. A patch of russet is clustered around the stem well.
The apple is attractively lopsided, like some modern objet d'art, with one great bulgy shoulder hunched over the smaller side. It has noticeable ribs. The "eye" at the very center of the base (the calyx, actually) is open, a tiny brown five-pointed star. The fruit is firm and has a faint sweet grassy smell.
Newtown's flesh is a crisp creamy white with yellow highlights, fine-grained, and juicy. The flavor is tart and moderately acid balanced with sweet, with hints of lime and cider. The overall effect is astringent, lively, and refreshing.
Promoters sometimes represent other apples as an American version of the popular Granny Smith. Although no one seems to have made that comparison with respect to Newtown, this is the only variety I have sampled that has anything like Granny's fundamental tang and crunch.
Many sources recommend letting the taste mature; these notes are from a fresh-picked apple.
Twinleaf Journal, the publication of the Thomas Jefferson Center for Historic Plants, has an engaging history of this apple redolent of tricorne hats and Yankee Doodle. A short meditation on the Newton Pippin (clearly the same apple) from the 1993 New Yorker magazine is also entertaining.
Update: This apple has its own advocacy group.