What color (colour) is best for you?
It's a matter of taste. Back in the old film days, Ansel Adams thought that technically, the camera bag ought to be white on the outside to keep the film cool in the desert and black on the inside to keep bright sunlight from fogging sheets of film in a film holder.
Today it is a matter of taste. The sand (or tan) bag shows dirt on the outside and looks bigger, but there is more contrast with the black lens caps, cables, sensor, lens hood, filter, battery, data card, etc. so you can get to the , little things at the bottom of the bag and avoid dropping an expensive lens onto your car keys and scratching the lens.
Black makes the bag disappear, become a shadow on your side and not call attention with shooting in crowds. It looks good whether you are covering a wedding or football.
The Navy color was offered because many people love blue and simply liked the color.
The brown and new stone washed looking bags try to be both. Dark enough so that it doesn't stand out or show dust, but contrast with black lenses, strobes, etc, so you can find things inside the bag.
My favorite story is how some high school students were visiting Europe and the train they were taking stopped at the border between Romania and Italy. Everyone was told they had to get off and to leave everything in the train. It turned out this wasn't customs inspection, but robbers. They looked through all the suitcases, but thanks to the worn beat up brown Domke Bag, they didn't bother to open it. All the students passports were in the Domke Bag!
I also heard that in a war zone, the green Domke Bag looks more military and soldiers will want to look inside it. Photogs are stopped more than those carrying a black, navy or brown bag.