Dell U2413 Monitor via DVI/HDMI to iMac 5k

I have a Dell U2413 monitor connected via Mini Display Port/HDMI to iMac 5k and the Dell display is grainy/fuzzy. Can anyone help in advising if this is a Thunderbolt video output config issue and if any config change can be made?
Appreciate your help guys. I have the Dell set to native resolution with no issues when it was connected to an iMac late 2010, which leads me to think this may be Thunderbolt video config related?

Everything I've read from Apple states that your 5K iMac can run at its full resolution and still be able to support an external monitor such as your Dell at its full native resolution. If possible, try another cable/adapter combination. A grainy fuzzy picture on the Dell could be the result of a defective or incorrect cable combo.

Thanks for your response chscag. Different cable/adaptor did not resolve the issue. Further searching on this suggests that the issue is related to YCbCR versus RGB Mode and a script is required to force RGB on a iMac 5k Mac OS when using External DELL Monitor
I am yet to source a suitable script for El Capitan to achieve this, but once done I will post solution if it worked for me to help others on this Forum.

Update....... There are a number of solutions on the net recommended for this to resolve the YCbCR versus RGB Mode, but I was reluctant to install the scripts as recommended. Further trial and error led me to change the Mini Display Port adaptor to HDMI with a DVI-D to Serial adapter and hey presto all fonts now crystal clear with no fuzziness and monitor appears much clearer. Hope this anyone else who comes across same issue.

Dell u2413 shows low resolution, and causes sleep wake failure.

Thanks for posting your solution, mickahtec. If you are still out there, I would be very grateful if you could clarify exactly what connection worked for you. I have the same kit (iMac 5k purchased last December) and the same problem (the monitor images have low resolution). Also when the monitor is connected via my mini dp to hdmi cable, it causes the iMac to wake from sleep at random times, producing a blank coloured screen, and mouse and keyboard are non functional.

Possible solution of the fuzzy monitor issue

I've had the same fuzziness problem with the Dell U2413 connected to both a Mac Pro and a 5k iMac, and the solution that worked for me was changing the Dell's 'color settings' to:
Input Color Format: RGB
Gamma: Mac
Preset Modes: Paper

No idea of the technical reasons why it works, but it worked for both the Mac Pro and the 5k iMac!

I had a rummage around on the Dell website to see if I could find any drivers for Mac, but all I could find was a message saying that Dell don't support Mac. Wish we'd known that before buying 40 of the blummen things for our labs!