As we all know, Kovu has no relation to Scar, which can only lead us to believe that his real father had similar colourings.. and now I can`t stop wondering why/how Vitani looks the way she does.

I'm thinking Kovu and Vitani's father was darker colored like Kovu, and from a genetics point of view, Kovu got his father's coloring, and Vitani got a lighter coloring like Zira. Well, Zira isn't very gold, but she's light compared to Kovu.
I think that even though Kovu and Scar aren't related, it made the story more interesting for Kovu to be darker like Scar so they could compare them.