Can someone who is colorblind watch a 3d movie?

Not a problem. Most color blind individuals have normal acuity and normal stereoscopic vision. They might miss some of the color spectrum differences but they will see the depth. you might be thinking of the type of 3-D (not used any more in movies) in which a red/green or a red/blue eye glass is used to see the depth. This will still work for a color deficient viewer.
Yes they can. Why not it will not effect threed, it may be perceived in different colour.