1. Get a random pic.
2. Grab a color scheme from your favorite color scheme generator.
3. Try to recreate the image using only those colors. In Photoshop, you can pick directly from your swatch or from the canvas. You cannot use the color picker. You can use the HSB sliders, but only the S and B sliders, and you can only move the S slider from more saturated to less.
Don't use colors in equal proportions. One color should always dominate.
Getting value right is still a top priority. Check in greyscale early and often.
Use more grey and fewer bright colors than you think you need.
To easy? Try again with the same pic and a different color scheme.