I'll split my answer up into two parts, since two different matters here are kind of intertwined.
Color Accuracy: Both the 21" and 27" displays on the mid-2010 models follow the sRGB color space standard and thus ITU Rec 709. White balance however differ quite a bit even between iMac units, so for pro/prosumer photo work on these iMac's, a color calibration tool is recommended. Compared to many screens on the market now, Apple monitors however belong to those with least color errors. One can often get better white balance by turning down the brightness, since turning it all the way up tend to make it bluer (higher color temperature).
"Vibrancy": As for "vibrancy", the only thing I've read that suggests a difference between the 21.5" and 27" models (if one would compare with those) would be that the 27" supports a higher brightness of 400 cd/m², while the 21,5" models "only" reach 330 cd/m². At seven "dots" in the display control, it's at 160 vs 130 cd/m², respectively. If the screens run at maximum brightness in a well-lit Apple Store, I can definitely picture this difference as making one see the colors as more vibrant.
This info is from a recent review in Swedish MacWorld.
