I recently experienced the identical problem with a new D620 purchased for my business. Grainy, shading and colors were off, generally poor. After one email to DELL Support they had me confirm if the issue was in the display or the video card by connecting an external monitor. Since the graphics on the ext monitor were as expected, they scheduled an onsite service call to replace the display entirely. The whole issue, although inconvient, was resolved in 3 days. The new display is excellent.
So although it was unfortunate to receive a defective machine out of the box, I am still a fan of this model with it's features and overall build. As a side note, I have purchase 2 additional laptops of this model since that first one with no issues.
Not sure what service option you got when purchasing your model, but if you feel like the balance of the system meets your expectations take the time to have the display replaced. It's a good system.