What do some fiber change the color of light?

Actually, all fibers change the color of the light in one technique or other. Because of the physical characteristics of the conductor a few frequencies travel with fewer impediments than others and this is not possible to produce a fiber which would have similar attenuation on the complete of the visible spectrum. For expecting a light conductor to transport millions of diverse wavelengths along with exactly similar attenuation in every one would be rather unreasonable.

Several fibers absorb less green than yellow and a little more blue than red and others just the opposite. Therefore, the hue and tone of the light varies from meter to meter, in several cases very apparently. This phenomenon is termed as selective spectral absorption.