Watercolor is fun and can make beautiful results. It's a transparent (or translucent) medium, and so most books and common knowledge about watercolor says that you must work from light to dark. Light colors won't show up very well over dark colors due to the transparency, and so that is the stated reason. But it seems that this advice comes mostly from books and articles about watercolor that focus on the traditional workflow of purist watercolor. But like most people, I've come to learn that it's best to ignore the rules and find out for yourself. There's actually a few reasons why you may choose to ignore this rule and work in either direction, light to dark, or dark to light.
First, it's more about "transparent" vs "opaque" than it is dark vs light. Famous artists like Paul Bonner make stunning images out of watercolor, except Bonner uses white gouache along with transparent watercolor. Since gouache is opaque watercolor, he's free to use it as a way to control the opacity of how he