Over the thousands of posts I've read about Zaino, the prevailing opinion is that Z5 works a little better on dark colors and Z2 better on light colors. I've used both products on all kinds of colors and personally can barely tell the diff.
You can see swirls in a light colored paint, they're just not as obvious but they're there. Correct prep procedures apply to all colors.
I have noticed an improvement in swirl filling ability from the new Z5 over the old formulation. Whether that's worth a few extra $$ is up to each person, of course.