Why Hating on Your Natural Haircolor?

Around 3 years ago, during the preparation of my sister's wedding day. I accompanied her to this hair salon where she got her hair and make up set for her big day.
We were there for make up test, trying on the dress, and hairdo consultation.
The stylist kept asking her to dye her hair a bit lighter. (FYI: my sis never colors her hair. Her hair is like, perfect black, very healthy.)
She refused to do it. But the stylist still said; it's better to have lighter hair color. Then we asked, "Why?". She answered,

the light color gives dimension to your hair. When you pull one section across another, you can see the depth, and it will looks nicer. While black hair, will appear with no depth, one dimensional, and heavy.

Well yeah, if I look at those Japanese Hairstyle magazine, almost everyone inside has at least brown-ish kind of hair color.
Me personally, at that time, had a red-auburn color. I keep dying my hair, because I have gray hair that's growing like crazy. Gray is contrasting so much with my black hair, so I decided on red or light brown to 'camouflage' them.

My sister is a woman with a strong will, stubborn I might say for sometimes. So she decided "No, no matter what, I'm keeping my beautiful black hair."

In another occasion when we were together having out haircut at a Japanese hair salon in Jakarta, the hair stylist also kept telling her to dye her hair.

What's wrong with keeping your natural black color hair?  Personally, I simply get bored looking at my dark hair. So, I experience with different color.
But for this past year, I've been dying my hair back to black. I'm loving it. See no problem with those "depth" issue. Except for my gray hair will be noted right away. But that's just a minor problem, I'll just cover them with the black dye again.

What do you think?