For many years black phones had black line cords, green phones had green line cords, and even pink phones had pink line cords.
But around 1970, AT&T realized that they could save millions of dollars if all telephones were equipped with neutral silver-gray line cords, regardless of the color of the phones they were attached to.
And since AT&T was a monopoly, customers' complaints could safely be ignored. Later on, when Ma Bell got some competition, most phone manufacturers followed Bell's profitable example since that's what people were used to, and the tradition continues today. Ironically, a $10 kid's phone is more likely to have a color-matched cord than a $400 CEO's phone.
In many cases, particularly if the back of the phone is against a wall, it makes absolutely no difference what color the cord is between the base of the phone and the phone jack, because nobody sees it except when the furniture is moved.
However, if a phone is away from a wall, and positioned where visitors see the cord when they enter the office, a light gray cord on a black phone sticks out like a sore thumb. Spending a few bucks on a matching cord will restore esthetic harmony.
Your feng shui consultant and interior designer will be very pleased by the improvement. We don't have a dozen colors, but we have enough to make a difference.