In DSSc the vital role is of Dye, which is degraded at high temperature because of its organic nature. Similarly, in UV light there is photo-catalytic degradation of dye; as TiO2 is active photo-catalyst.

Additionally, different dyes have their own peak absorbance depending of band-gap etc. For N-719 red colour is better absorbed as compared to other colors; you can google the absorbance spectrum of N-719. So, N-719 performs more better in light having more percentage of red color as compared to blue color.

This behaviour will always be seen in any cell that has significant series resistance. The less current it has to supply, the less voltage will be lost inside the cell and the higher the efficiency will appear to be. But because the overall light level is less, so the actual output power of the cell will be less.