Hold on, I just read something interesting:

"In the story text he is described as golden furred with a black mane and green eyes whereas the pictures show that he is like Mufasa in both build and colour." -Wikipedia

Apparently, he is both! It really depends on what you believe. As stated above, he is descrbribed as having a black mane, but pictured with a red mane. So you really get to choose what he is, because it is unclear as to what mane the creators made him have.

Here is the text from the book:

"Rafiki looked up and saw a lion on the rocks above him. He had a long black mane and green eyes. It was Ahadi, the Lion King"

And a link to an image:

http://images4.wikia.nocookie. [...] ges/b/b3/Trouble_for_Ahadi.png

In the book, he has a red mane. That's the official version of Ahadi by the creators. The black mane is a fan-made version. Scar's black mane may have been inherited by Uru, maybe her someone in her family had a black mane, such as her father. But there are many, such as myself, who think that Mohatu, who also had a red mane, was her father. But really, the gene could be on either side. Just because Ahadi had a red mane doesn't mean his family doesn't have the black mane gene.

Just because uru doesn't have a mane doesn't mean she didn't pass on the dark-haired gene to Scar.

well uru does not have a mane

Mufasa most likely got his mane color from Ahadi, and Scar most likely got his from his mother, Uru. Uru is a dark lioness.
These are again unknown, but this is most likely the genes.

in the books it is redish brown, but I personally think he looks better with black mane ^^ However, the true color of his mane is most likely red, but it is a bit unknown of his true mane color.