From Yokohama's web site:
"....the red mark on the tire, indicating the point of maximum radial force variation, should be aligned with the wheel assembly's point of minimum radial run-out, which is generally indicated by a colored dot or a notch somewhere on the wheel assembly (consult manufacturer for details)......"
Please note that this only appies to Yokohama tires. Other manufacturers don't necessarily use red dots - some don't apply any marks at all.