Can you regain the ability to see color if you are color blind?

No. Unfortunately, at this point there is no cure for color blindness. The exciting news is that jay neitz, phd at the university of washington was able to introduce the missing gene for color blindness into the retina of color blind monkeys and cured the disease. The monkeys responded to colors. Time magazine listed this as among the top 10 scientific discoveries in 2009. So there is hope!
Mostly no. Most color deficient persons got that way from an inherited, and therefore lifelong condition. There is a lens called the x-chrom lens, which allows some color deficients to pass the standard color recognition tests, but these simply shift the spectrum making them color defective in a different way. Some with optic nerve disease lose color sense which can return when the nerve heals.
No. Several varieties of color blindness exist, but they are permanent conditions. A person is born color blind because he has a mutation in the genes that are needed to see certain colors the way a normal person sees them. Most color blind people see many colors, but the colors appear in a "different" hue. Very rarely does anyone have total color blindness and see only black, white, and gray.