Let's take the color orange for example. Print a sheet with 5 different shades of orange, with each shade having progressively greater amounts of white mixed in with the orange. Then apply the test piece to a metal enclosure and see which shade most closely resembles what it looks like on the computer screen.
The gray metal enclosure is going to dim down the color so the idea is to put white into the color to bring back its brightness. Then find which shade of orange and white looks closest to the design you want once its on the enclosure