Alternating the colors helps a lot with determining where you've painted, but it is also very useful for helping you coat the areas around the boat stands. For instance:
The first layer is gray, since the gelcoat is white, and you can paint
right up to the boat stand pads. Then you paint
a layer of white, and leave about a two-inch margin of gray paint
around the pads... then paint
a layer a gray and leave a four-inch margin around the pads or about two-inches of white and two inches of gray showing...and then finish with a layer of white—with a six-inch margin around the pads—with two inches of gray, two inches of white and two inches of gray.
Then when you move the boat stands, you can fill in the pads and layer the paint
accordingly... adding gray to cover the white square left by the pad.. then white to cover the gray square, and so on.
Also, by alternating colors, you can see if someone has sanded through the barrier coat when you're prepping the boat for re-painting. If there's an area that is gray or grayish, they've sanded through at least the outermost layer of barrier coat. If you had all white, you wouldn't be able to tell if they had sanded down through the barrier coat as easily—if you had all gray, you could tell they sanded through the barrier coat...but not if they've sanded into it...
I hope this helps clear things up a bit.