Correct. The Built-in Diagnostics passed.

* To drive the 5K3K 5120x2880 resolution, the video card must have two DP outs or two mDP outs to connect to the UP2715K DP #1 and DP #2
* The video card two DP or two mDP must support 2x DP1.2 and have DisplayID v1.3 support
* The video card driver must support 2x DP1.2 configuration with DisplayID v1.3 support
* The video card driver is responsible to match up the dual 2560x1440 side by side tiles to create the 5K3K 5120x2880 resolution.
* Validated video cards =
Quadro K5200/K4200/K2200
GeForce GTX 1080/1070/1060/980/970/960
FirePro W9100/W7100
Radeon R9 295X2, HD7990

I just ran its stand-alone Built-in Diagnostics and the colors are normal, not split. Does that mean the monitor is fine?

Unfortunately, the monitor was purchased second hand and there was no invoice. And I tried to contact the shop it was purchased but they also do not have the invoice. Probably a company sold its all computers second hand after a bankruptcy. 

I am having issues with my Dell UP2715K Monitor. Half of the screen (right side) is darker than the left side.
* I need to see pictures of the colored screens while running its stand alone Built-in Diagnostics, page 52. Note, we do not have 100% full screen color uniformity guarantee. Our guarantee is that the 5 color zones by themselves will meet our manufacturer specifications.

I am not able to find the invoice for the purchase.
* Did you purchase the UP2715K directly from Dell Turkey?
- If yes, then you have a Dell customer number and Dell order number. Using those, Dell Turkey Support should be able to find the original service tag number.
- If no, then you need to contact the reseller and get a copy of their invoice.

I will contact them with this new Dispatch Document. 

I am sure there must be a way to find out what was the previous Service Tag with the Service Tag Number on the replaced monitor's box. It is a simple record Dell would/should keep in such cases, right? Also, I did not ask for a replacement to Dell Turkey. They simply said there is nothing they can do which means I can throw this monitor to garbage?

As for the cabling, as I mentioned, I tried it with different cables and computers, but the result is the same.

Is there any setting on the monitor which would make the monitor seem like this?

https://www.dropbox.com/s/26k2ou121w0gmhx/20180814_160329.jpg?dl=0

Regards

This is a monitor we were using it with a computer for more than 1.5 years. We did not change the configuration of the computer.

The video card is NVidia 980Ti which has 2 DisplayPort 1.2 and we are using them as we were using before. So nothing changed actually. When we switched on the computer it was like this.

https://www.dropbox.com/s/n1jdnc7tqe33l6t/20180814_105909.jpg?dl=0

https://www.dropbox.com/s/zxsjl3r59wgjv61/20180814_110245.jpg?dl=0

https://www.dropbox.com/s/26k2ou121w0gmhx/20180814_160329.jpg?dl=0

As you can see it is the same even on the DOS Screen.

I also checked it with another computer, with a single MiniDisplay Port from an NVidia 1050Ti Video Card, but the result is the same. The right side is darker than the left side. 

I found a dispatch document in the monitor's box with a Reference Number: F1JCM-CN:32317825459

Might this be the Customer Number?

Regards

Interestingly,

"Reset Other Settings" under the OCD Menu/Other/ corrected the issue!.

I am not sure how.

Regards

Hmm, that is interesting. I wonder if what was turned on in there that is now disabled.

The part I don't understand is what are the "Other Settings" (and where) which result in a screen with different darkness.

Menu- Others- Reset Other Settings only changes =
Auto Rotate
DDC/CI
LCD Conditioning
So you must have had one of those set to something different.

Menu- Others- Factory Reset- Reset All Settings resets every category to factory defaults.