To sum up this approach: "Light" colors are those that are, well, light hues of a particular color (for example: light blue) and "Shadow" are those darker hues of that particular color (Navy). If a color scheme contains a lighter hue of a particular color adding a little "shadow" (i.e. the darker hue of that color) to the scheme gives the scheme some depth and variety while ensuring cohesion.
The same strategy can be applied if the space has a darker hue ("Shadow") by adding lighter hues gives the space some "Light" and that depth and variety the scheme is missing.
There are a number of ways to add "Light and Shadow" to a room but one fool-proof way is to introduce accent items that contain both the light and the shadow of a particular color and then expand from there. Here a few of my favorite items that contain my preferred colors: red, orange, green and blue and offer some "Light and Shadow":
1. West Elm Phoenix Wool Dhurrie Rug
2. Blue Dot Charlie Rug
5. Target Room Essentials Chevron Duvet Cover.
6. Crate and Barrel Marimekko Tikkula Seaglass Sheet Sets
7. Target Room Essential Ombre BLock Sham-Green
8. West Elm Lourdes Sanches Bull's Eye Silk Pillow Cover
9. West Elm Chain Crewel Pillow Cover
10. West Elm West Elm Abacus Wool Dhurrie Rug