Light colors can be layered over dark colors and still show up.
Light colors will not show up over dark colors.
To make pastel colors, you have to add white to the color.
To make pastel colors, just add more water.
Since watercolors are transparent, they have a luminant effect and almost seem to glow.
Gouache uses less water, so it dries much quicker.
Watercolor requires a lot more water, so it can take longer to dry.