As you may know, watercolor is a transparent medium that works from light colors to dark ones. That’s why you need to think and paint in layers!
How does this affect you and your artworks?
The brightest points of a painting will be given by the white of the paper, basically the absence of color.
You need to build up the painting from the brightest layer to the darkest one (usually carrying the details). If you worked in reverse order (this happens in opaque mediums, like acrylics) you would not be able to obtain any points of light over the dark parts.
Oh… And remember to let all the previous layers dry very well before starting another one on top!
Otherwise you will end up with an unwanted mix of colors on your page. Unless you are doing it in purpose, you want the dark color to cover up the other layers and not to mix with them.