Did you just received your mini?
Me and my girlfriend just received ours on 4th April fromonline Apple Store.
Her set was slightly yellowish on the edges, very close to the bezel. Not really noticeable, so she decided to keep it.
As for my set, I could see that the lower half of the screen (portrait orientation with home button below) was extremely yellowish. This was further aggravated by the fact that there is a spot of lighter colour on the bottom right corner, about 0.5cm in diameter. This creates a very visible constrast on a light coloured background (I woudn't be able to see a white spot on a white background otherwise). Immediately called Apple to do an exchange. Now waiting for the courier to come collect my mini.
I'd advise you to push for an exchange too if it bugs you.