Spraying the lightest color of the camo as your base would only work if you're going for a lighter camo design, such as the Desert Strom camo.
This was the order in which I did mine in order to achieve a more comventional camo look ...
1. Olive Green as your base
2. Then brown patterns
3. Then khaki patterns
Had I not used the color black for stripes, I would have then used black for my third and final patterns.
Guess it really all depends on the look you're going for and which camo design you want to achieve.
Good luck.
It can be done either way of course. The reason I use the lightest colors and go darker is an issue of coverage. The darker colors need less coats to achieve the color so you get less of an edge when unmasking each color. This saves tim in the clearing stage when you are trying to bury those paint edges.
I have done it both ways and the design does indeed dictate the best method.