After cataract removal and iol insertion there is a blue, faint fog like, distortion in my field of view if the background is yellow. and brighter blue if the background is blue. what is the cause of this? will this improve? already 3 weeks since?

According to my reading cataracts eventually cause a blue light blocking effect and the brain loses its ability to distinguish blue hues. This returns with emphasis after cataract removal. Eventually both eyes will distinguish blue color the same.

I would tell this to your surgeon, any distortion in the center of your vision could be from post surgical macular edema and should be diagnosed and treated.