They are just gamma presets for PC or mac.

Use the monitor OSD (On Screen Display) and reset it all to factory defaults.
* Press monitor Menu button to open the OSD
* Go to Other Settings- Factory Reset
* Choose Reset all settings

Run the Built-in Diagnostics and check the colored screens
* Turn off both your computer and the monitor
* Unplug all video cables from the back of the monitor
* Turn on the monitor
* The monitor then goes into the self-test mode
* Looking at the monitor buttons top to bottom, press and hold buttons 1 and 4 down for two seconds
* An all Gray screen appears
* Press button 4. The screen changes to Red
* Press button 4. The screen changes to Green
* Press button 4. The screen changes to Blue
* Press button 4. The screen changes to Black
* Press button 4. The screen changes to White
* Press button 4. The screen changes to Text
* Press button 4 to exit the test
How did the colors look?

Did you go to Custom Color and calibrated with Dispcal?

PC and MAC gamma: I asked Dell in the past and the answer I got (as usual Dell Support is very helpful...) was "We cannot answer because we do not support Mac".

My understanding is that this is a legacy switch from the time when Mac di not run on gamma 2.2 like PC. Since Mac has transitioned to 2.2 I always set that on "PC" even when using a Mac.

I found that pc gamma is too low. Black looks really too black so I found pc gamma is total useless for me.

So on all grey screen there was stange screen noise .... Horizontal lines all other colors looks not bad to be honest

I would not do that then. Did you calibrate the monitor? I had this monitor and once calibrated it was great.

Sorry I can't help more but I am not an expert.

is it normal  PC gamma to be much darker then Mac gamma ? when using monitor on PC setup... its rly strange to me because when i switch to pc gamma all is darker on mac gamma seems fine , anyways thanks for answering i will buy new video card tommorow and see is there are any differences. Maybe monitor model itself is good i red many many positive commentars and overviews for this monitor, maybe just mine copy is broken 

If the colors look better when running the monitor Built-in Diagnostics, then the monitor is ok. The issue is the computer, video card, video card drivers, or the cabling.

im using PC , but gamma settings are set Mac

No i didnt calibrate it even i dont know how to do it, but i cant accept that preset modes from dell are not accurate and need calibration, also i will use monitor for gaming , not for graphic desing or photography or something like that

Unknown. We only test using the default monitor settings, which is PC gamma.

There is again me ... so i had brand new u2412m ... is there any way to increace black lightning on PC gamma because all loocks rly dark and  unreal on black/shadows etc its unplayable any game... i think mine problem was old video card today i purchaced brand new GTX 960 and on pc gamma all is dark  on mac gamma colors are not real... so i thing problem is black lighting of pixels or wathever is called