Yes, we can still plant in zone 6 in October! Crops that we can plant now are called overwintering crops (aka crops that can tolerate and survive the winter weather). The success of these crops depends on many factors. The most important ones to consider are the crop variety, the timing of planting, and the provision of proper protection.
Overwintering crop varieties include endive, arugula, spinach, kale, beets, parsnips, carrots, and radishes. When you’ve decided to grow any of these crops, look for plants that are specific to being more cold hardy. For example, spinach generally is a cold hardy crop but the deep crinkled types like "Tyee" or "Winter Bloomsdale" are more cold hardy than flat leaf varieties.
October in zone 6, is usually the last month to plant anything for overwintering. The timing of planting is still beneficial because plants will still sprout from seed or grow from transplants due to warmer daytime temperatures and cooler night protection from a hoop house or cold frame.
As the weather gets colder, plant growth slows yet plants are still protected from the harsh environment because of a hoop house or cold frame.
Once spring arrives with increased sunlight and temperatures, plants will come back to life and start growing where they left off. It's a double bonus due to earlier production and a longer growing season... score!