No surprise: It's by temperature, not color.
When making caramel, we recommend using an instant-read thermometer to gauge when the sugar has reached the appropriate degree of caramelization called for in the recipe instead of simply eyeballing it to evaluate if it's dark enough. That's because the color of your cookware, the lighting in your kitchen, and especially the amount and depth of caramel you have in the pan will affect its appearance.
To prove the point, we caramelized two batches of sugar according to our recipe, taking both to 370 degrees. We made one batch in a wide skillet and the other in a small saucepan. Just as water at the shallow end of a pool appears lighter in color, the thinner layer of caramel in the skillet appeared lighter compared with the deeper layer in the saucepan, even though both vessels contained the same volume of identical caramel.
BOTTOM LINE: For the best results when making caramel, don't rely on color alone. Use a thermometer to determine how much the sugar has caramelized.