I posed the same question at some point last year. And now, if I'm not using filters, I always use a lens hood. If nothing else, it acts like a crumple zone when I drop the camera!

I have only just started using my flower hood cap on my 18-55mm and I can see the colour differences already and I am not as paranoid about scratching my lens with it protecting it.
Just think about how you cup your hands over your eyes when looking into the distance on bright days - exactly the same principle, you're eliminating scattered light to see things more clearly.