In my mind a space marine is not exactly what I would call stealthy in the first place so why bother concidering most people have the ability to hit somthing the size of a barn door anyway, but they are shock troops so bright colours makes sence like blue woad paint on ancient Britons as a real life example.

Any guard regiment that uses brightly coloured uniforms does so becuase they are awesome and that's what wins wars.