I am in zone 9b and growing tomatoes since summer in a vegetable garden. I have a few tomatoe plants that have small tomatoes. Now that the weather has started to get cold, the tomatoes are not turning red as you would normally expect. How do I ripen them or get them to turn red? Should I bring them into the house where it is much warmer and let is sit till they turn red?
Unless they're too immature, they will eventually turn red if you bring them in the house and wait. If you're planning to save seeds, though, people often say that the seeds cease to develop after you've picked them. Nevertheless, seeds from full-sized unripe tomatoes (in my experience) do grow. If they don't look very developed, or if they're particularly thin, I wouldn't count on them growing, though, whether they're green or red.
In some rare cases (in my experience), green tomatoes that are supposed to change red won't fully turn red if ripened indoors (but they will change yellow or orange). In the vast majority of cases, however, they will turn red. Either way, they'll be ripe and you can eat them. But it should be noted that just because an occasional one turns yellow, that doesn't mean the seeds will produce yellow fruit (because it's not cross-pollination that causes this). I forgot the reason this happens, but I did read about something like this once.
Green tomatoes, in my many years of experience storing them to ripen, can take anywhere from a day to several months to turn red indoors, depending on various factors (including the variety). Contrary to popular belief, you don't have to get special storage tomato varieties to do this (whether or not they're any better for the purpose). Most tomatoes will work. Some ripen fast, and some take a long time to ripen (the ripening speed outside is a good (but not perfect) indicator as to how long it will take).
Tomatoes will usually ripen faster if they're warm. Refrigerating them isn't a good idea, but dry, cold storage is great if you want to delay ripening so you can have tomatoes months from now, or what-have-you.
If your tomatoes have a dull look to them, or aren't smooth to the touch, they're probably not ripe enough to be worth storing indoors. They can ripen still, but they may end up being leathery.
Ethylene helps tomatoes to ripen faster. It is found in other ripe fruits. Putting other ripe fruits near your green tomatoes may speed the ripening process; however, it may also make them go bad faster after ripening.
Phosphorus is found in large amounts in seeds, and it is also limited when it's cold. If you still have a decent amount of time left before the frost, and if your plants are still warm enough to actually be growing bigger, then you might try giving them some extra phosphorus to see if that helps the seeds to mature (which may in turn help to speed ripening). Phosphorus seemed to help with my melons last year, anyway (I didn't try it this year). I have read that phosphorus helps to speed ripening, but it took a lot of searching to find anything on the topic; so, I would consider it experimental, personally.
If you try it in future, you'll likely find that some varieties are a lot better at ripening fast even on slow-ripening years than others. This was a slow-ripening year for most varieties for me. I grew about 100 varieties. For me, the faster-ripening ones (after fruit set) were Chocolate Pear, Matina, Creole, Aussie, Pink Cheeks, Mountain Princess and perhaps some others.
Unless you have a small variety of tomatoes, they're likely to be immature and will never ripen off the vine.
If they're mature and still green, then they will ripen off the vine. Keep them in the dark covered with some newspaper to minimize the loss of ethylene, at a temperature of 68°F to 77°F. Outside this range the ripening slows down, or completely stops.
Once they're off the vine the sugar levels will not increase any further since the plant can not store any more sugars in the fruit.