Original: These colors are either paired with a color representing a metal (often black, brown, gray/silver, white, or gold/yellow), or occasionally each other, such as orange/green, blue/orange, or blue/red.
Simplified: These colors are either paired with a color representing a metal (often black, brown, gray (or silver), white, or gold), or occasionally each other, such as orange/blue, red/green, or blue/yellow.