With the help of Club 3d I managed to make it work eventually !!!!
My problem was that switchresx has a bug and stops saving resolutions and I was trying with the ones which were already in switchresx (4k@60hz) BUT they didn’t had CVT-RB on.
I created a new user in OSX and switchresx started to save my custom resolutions.
Now it works with ultra hd on, when ultra hd is off, colours are pink.
My solution was to make sure I use a proper cable (I have tried with 3 cables and it didn’t work, they were all advertised as 2.0 but they weren’t), and make sure you make a custom resolution 4k@60hz with CVT-RB on.
This is a picture with my settings.
Good luck lads !