You can't go by color. Engine oil contains dispersant; additives that pick up and hold degradation products in suspension. They also carry foreign entry substances (like soot entering a diesel engine crankcase.) If not for dispersants, the "stuff" would settle and cause sludge puddles inside the block, restricting oil flow back to the pan, occasionally being dislodged and blocking the oil screen, and generally causing problems with the life of the oil. Having dispersants, and changing your oil periodically, ensures that the inside of the engine stays clean. You could say that a better oil turns darker sooner than a lesser oil. The right way is a used oil analysis looking for wear metal levels, glycol, and viscosity.