Both of them are possible - lower quality color reproduction and degraded touch sensitivity. Check out the reviews from those who have actually used the screen protectors.
I do not think there is (or will be) a consensus as to if screen protectors are necessary. It is up to you. I personally do not use them, but if I must I would choose protectors made of glass. They used to cost a lot but nowadays their prices are more reasonable. As long as you purchase high quality protectors (again, check out the first-hand experiences) you should not have a problem with colors and touch sensitivity. But many people use tablets without screen protectors just fine as well.