How do i get rid of "Posterize Effect" on MacBook Pro?

I am running OSX El Capitan (10.11.5), and for a few weeks now i have been getting this "posterize effect" (where the shadows appear purple and some highlights as well).

I have found that putting it to "sleep" and waking it up right then, it takes the effect away. but now its taking me 5-10 times to get rid of it.

I have also did a hardware check, and all was reported ok.

could it be the video card? there is no way of reinstalling the drivers and they are all up to date.

any help would be great!

rodrigo