To be completely honest with you, I haven't been paying attention but I can see where your theory comes from. To me, I don't think there's a particular association with hair color and a side couple. Take Brothers Without a Tomorrow for example: I know that Blood Link has a side story and a side couple, but that came from the fact that the two had a kid and, possibly due to popularity, the kid received a story of his own as well as the other side couple. In Taming the Tiger, both leads have dark hair colors but don't have a side couple. In Low Tide in Twilight, the top has light hair but the author just announced a new story featuring his parents.
I'm not a publisher, but from my limited knowledge, I believe this has to do with popularity and fan demand. Love is an Illusion for example was the author's most popular work. In order to gain money from the work's momentum, I would assume they continued milking the franchise for what it's worth. As for your claim about ukes looking the same, it also comes down to fan demand and art style.
In conclusion, I believe it's intentional per author, not in general. And if it's been proven through multiple works, I also believe it's because of the money-making aspect and popular demand. Thoughts?
I've noticed a pattern in manhwas featuring beefy, dark-haired bottoms—like Black and Blue—where there are practically no side couples with a muscular top of the same hair color.
But in manhwas where the main top has dark hair, there’s almost always a side couple, and the bottom looks like a near-identical copy of the main one.
It’s too consistent to be a coincidence.
Are they deliberately creating certain dynamics?
Trying to challenge the usual straight manhwa tropes? Is there some deeper agenda at play? Hard to say, but it feels intentional.
Side note:
I read this story—it was decent, but I expected another couple