The appearance of colour in solid alkali metal halides is generally...

The appearance of colour in alkali metal halides is generally due to F-centres. F centers in $\mathrm{NaCl}$ produce a band of optical absorption towards the blue end of visible spectrum, thus the colour appears yellow under transmitted light. On the other hand, $\mathrm{KCl}$ with $\mathrm{F}$-centers appears magenta, and $\mathrm{KBr}$ appears blue.