Why fiber optic cables looks colorful?

I assume you mean these things:

They are lit up by the light source below, which can be any colour. In many of them the source, which is an LED, changes colour over time. The reason why the cables themselves look coloured is that they do not have total internal reflection. Some of the light leaks out through the sides of the cables.

Not having total internal reflection also means they would be useless for transmitting data over long distances: as too much light leaks out, not much ends up at the end of a long cable.