It helps if you have a picture where the bike color is different from any other colors in the picture (KTM orange on a green/brown background or Kawasaki on a tan/blue background), then just play around with the selection wand, or color selector until you get the majority of the background selected. You'll have to play with the threshold to get a good selection, but just be patient, cause it's rather easy. From there, I just turned down the saturation on the background all the way.
I'm sure there are other ways, but like I said, I was just messing around