The color of qsos shown on a U-B vs B-B color plane, with the colors of mains sequence stars for comparison. Overall, QSOs are blue, with U-B < 0 being a reasonably good selection criteria (since O & B stars are rare at high galactic latitudes).
The redshift distribution of the 1240 QSOs catalogued in 1993, most of which were selected by "UV excess" (i.e. U - B 
0). Note how the redshift distribution closely follows the U-B "K-correction". Above z ~ 2.2, the Ly-alpha emission line moves into the B band, so the QSO appears red and no can no longer be identified.