Back when I was first learning about hair color and the variations of gradience and dimension, I spent a lot of time thumbing through black and white photographs. By removing color from a photo, you are left with varying levels of depth within the black to white scale. A photo with zero color demonstrates true light reflection (the lighter aspects of the photo) and light absorption (the deeper, darker bits of the image).
If you were to take your favorite photo of hair and transfer it to black and white, your brain could measure the difference from dark to light. It would show how extreme the colors contrast with one another and how well the gradience of the hair "flows". My little sister has some of the best gradience and contrast I have ever created, so I took some photos of her on a recent trip to California and eliminated the color from the images.
Color, like anything, can be broken down to basic fundamentals. Next time you find an image of a haircolor that you like, I challenge you to make it black and white and study the contrasting colors of the hair in a simpler form.
Love you all!