It could show their friendship developing or present a challenge they face together. Maybe it gives us insights into their personalities and how they interact.
The JoJo manga is mainly in black and white. Colored versions exist but are not the norm. Sometimes, colored illustrations are done for promotional purposes or by fans.
I think it's great. The characters are memorable, the plot is full of twists and turns, and the battles are epic. It's a manga that stands out from the crowd.