# What values can a color index variable get?

I am a statistician with love and passion for astronomy. I try to explain some outliers I found in the values of quasars - color indexes. I found four (4) values close to twenty (20). Is that scientifically possible or there is an error in the CSV file? What values can a color index get?

• What colors are we talking about? In principle something line $U-V$ could get high if all UV light is absorbed (by neutral hydrogen), but that would mean an unrealistically large range in observational thresholds. Are these actual observations, or simulated quasars? Also, are we talking AB or Vega magnitudes? – pela Apr 2 '19 at 12:20
The only physical mechanism I can think of for quasars to have such a large difference in two bands would be at very high redshift ($$z\gtrsim6$$) where you have the Gunn-Peterson trough completely erasing everything blueward of 912 Å (restframe). But with typical magnitudes of $$\sim20$$ or so at these redshifts, a color of $$20$$ that would then mean that the magnitude in the bluest band would be of the order $$\sim 40$$, which no instrument is able to detect.