You just need some grease paints and a slight knowledge of colour theory. I used Make Up For Ever Flash Colours for this but Kryolan, Ben Nye and Grimas do brilliant versions.
1. Take small amounts of your primary colours. Red, yellow and blue plus small swatches of white and black.
2. Mix them all together so you get your base. It should look like mud!
3. Add more white or black depending on your skin shade.
4. Adjust with more yellow, blue or red depending on your undertones.
Voila! A custom made perfectly matched foundation or concealer for you or your client.