Pastels are excellent for combining flat design or other bold design choices (like sans serif fonts) because they don’t try to compete with that design element. Retro and nostalgia are perfect matches for pastels. Primary colors, huzzah! Pastel primaries allow you to embrace these bright, bold colors without overwhelming users.
And while we’re at it, ANY color, huzzah! If you want to use a ton of color, you can do so without overrunning other design elements (and overwhelming users). warms the picture up, making it more approachable and interesting. Contrasting real photography with pastels graphic draws attention where it’s needed most. Using pastels with bold typography can really make the text stand out, especially if it’s against a simple background.
With all that said, we’ve done some exploring on the inter webs, and I’ve found some sites that are killing it with their use of pastels. Each site brings forth something different about the use of pastels that might make even the most pastel pessimist swap out their favorite electric blue for a cool shade of baby blue.