Opposing points on the color wheel have high color contrast. In the diagram, yellow (A) and blue (B) have high contrast.
This is why a blue pan will help you to see gold more easily than all other colors. Black may seem like a good choice, but it will camouflage the black sand that is commonly found with alluvial gold.
Adjacent colors on the color wheel have low contrast, so orange or green would not be a good choice of color for gold visibility.
Low contrast combinations
Green and yellow are from the light half of the wheel and do not reveal gold nuggets easily.
High contrast combinations
Red and blue are both ‘dark’ colors, giving maximum contrast to make gold nuggets highly visible.