What you were told : “Try foundation on the back of your hand to test the color”
No way! Our hands are hardly ever protected from the harmful UV rays typically making them darker than your face. The correct way to test foundation is to take 3 colors that you think match and draw little stripes on your jaw line. The color that disapperars when you look into the mirror is the right color. Having the other colors is important for comparison.
Just a tip!