Why does rock salt change colors?

I'm not aware of it "changing" colors once it's formed, unless it's done somehow with food coloring.

If you're talking about natural salt crystals being a color other than clear/white, that happens as the crystals are forming. The cause depends on the color of the crystal. I know the ones that are pink are due to an algae and blue is caused by radiation and/or defects in the crystal structure. This website http://minerals.caltech.edu/COLOR_Causes/Radiate/i... says radiation also causes an amber color in rock salt. Some colors are caused by the inclusion of other minerals or elements as impurities.