To speed the ripening of tomato fruit on the vine, slightly reduce watering. With the forecast of a light frost, protect fruit by covering. If heavy frost is forecast or where covering is not practical, harvest fruit before the frost event and carry indoors.
Pick ripening fruit and green tomatoes with a glossy green appearance that have reached at least three-fourths of their full size. Remove stems. Wash fruit under a stream of water and allow to air dry on a clean towel. Save only blemish-free fruits for ripening indoors.
Fruit does not need light to ripen. One difficulty with ripening tomatoes indoors is controlling humidity. If the humidity is too low, fruit shrivel. If the humidity is too high, fruit molds. Some gardeners simply hang the whole plant upside down in a dark, cool garage or basement to let the fruit ripen gradually. In Colorado, fruit tend to shrivel from the low humidity.
Other options include placing tomatoes one or two layers deep in a covered box for ripening. Some people find better success by individually wrapping fruit in newspaper or wax paper and placing them in a covered box. Placing a few fruit together in a plastic bag has been effective for others. For higher humidity, place tomatoes up to two layers deep in a blanching pan or strainer inside of a covered pan with some water in the bottom. Make sure the fruit does not touch the water.
Ethylene gas produced by ripening tomatoes is a ripening hormone. To speed the ripening process, place a ripe tomato or banana in the container with the fruit. To slow the ripening of green tomatoes, routinely remove ripening fruit from the container.
Green fruit will ripen in about two weeks at 65 to 70 degrees F, and in about 3-4 weeks at 55 degrees F. Storage below 50 degrees F will give fruit a bland, off flavor.
For more information, see the following Colorado State Extension fact sheet(s). http://www.ext.colostate.edu/ptlk/1831.html