Initially the impression that Apple gave was that the iPad mini with Retina display was pretty much the same as the iPad Air, but smaller. Same processor, same high-resolution display, same camera. But small differences are emerging. The processor clock speed on the iPad Air is a bit higher than the retina iPad mini, and now AnandTech is reporting that the new iPad mini has the same color gamut as the first generation device, and a smaller range than the iPad Air. Their article shows the sorts of tests they run to measure a display's performance. The new mini clearly has a narrower color range. However, they said the difference is a non-issue and that while the difference is apparent, it's small. They say it would mainly be of concern to photographers and other professionals who need the best possible color reproduction.
To illustrate, they put side-by-side screenshots of an iPad Air and retina iPad mini with a highly colorful wallpaper and point to the color of the red triangles in lower left. Frankly, I can't see the difference, but they say it's there. Their tests show that the Google Nexus 7 and the Kindle HDX 8.9 also score better than the retina iPad mini.
Still they praise the display of the new mini, and say that it's superior to the previous model.