I'm a biologist and I use viruses (adenovirus to be specific) to insert genes into mammalian cells in vitro. I do this to study the function of said genes. To make a usable stock of virus, I infect a small number of viruses into particular cell line, wherein the viruses replicate and give me an 'amplified' stock.

When I harvest the amplified viruses, I use a centrifuge to separate them from the rest of the cellular crud. The viruses separate into a distinct, narrow, blue-white band in the centrifuge tube. It seems that concentrated virus has a distinct color, despite being made up of colorless viral particles.

How can this be so?