I think a lot depends on whether you spray, brush , or roll-and- tip. I just painted Bluestocking with white Imron, which included a new corrected waterline.
We sanded the previous LPU, rolled on 2 coats of epoxy
primer, and then 2 coats of Imron. Took it right down to the anti-fouling
White is popular because it can support a higher percentage of solids, and usually goes on over a white primer, so normally goes on first.
Blues, greens and reds don't suspend solid pigments very well, and therefore are often put on over grey primer to help with the opacity of the color.
For paint jobs of 2 contrasting colors, the tape line is often buffed out, and clearcoated.
The above, I learned from a professional painter, who worked with me on my boat.