What's the best color of light to read by?

The color of light is not as important as the intensity of the light for better reading. The ability of our eyes to accommodate (focus for near tasks like reading) diminishes as we age. This condition is known as presbyopia and it occurs to most everyone in the fifth decade of life. Patients who are nearsighted (myopic) will see a more gradual decline in reading vision. Brighter light helps with reading and the color of light is really not the major factor in reading better. No one light is better than another (white or yellow), yet a bright light is extremely important in reading which is why people with presbyopia read better during the day compared to at night.