Why are the K dwarfs in the Pleiades so blue?

The K dwarfs in the Pleiades fall nearly 1/2 mag below a main-sequence isochrone when plotted in a color-magnitude diagram utilizing V magnitude as the luminosity index and B-V as the color index. This peculiarity has been known for 40 years but has gone unexplained and mostly ignored. When compared to Praesepe members, the Pleiades K dwarfs again are subluminous (or blue) in a color-magnitude diagram using B-V as the color index. However, using V-I as the color index, stars in the two clusters are coincident to M-V similar to 10; using V-K as the color index, Pleiades late K and M stars fall above the main-sequence locus defined by Praesepe members. We believe that the anomalous spectral energy distributions for the Pleiades K dwarfs, as compared to older clusters, are a consequence of rapid stellar rotation and may be primarily due to spottedness. If so, the required areal filling factor for the cool component has to be very large (greater than or equal to50%). Weak-lined T Tauri stars have similar color anomalies, and we suspect that this is a common feature of all very young K dwarfs (spectral type >K3). The peculiar spectral energy distribution needs to be considered in deriving accurate pre-main-sequence isochrone-fitting ages for clusters like the Pleiades, since the age derived will depend on the temperature index used.