In the watercolor world the brightest light is the white paper, of course, but there are a few steps you can take to enhance the effect a little.
Contrasting a light area with a relatively strong dark makes the pale "sunlit" streak in this scene look extra bright.
When you surround a white space with a soft-edged, intense warm, like Quinacridone Gold, It glows even without a contrasting dark nearby.
If the lightest area in a painting is not pure white, it is a good idea to consider tinting it with a powerful transparent hue so you can make it very pale and still have a significant amount of color.
Here are a few photos that involve powerful brightness. Have some fun with them, and try to get your darks dark enough on the first try. Feel free to copy the paintings, too.