Start with understanding the basic anatomy of muscles. Study how they attach and move. Then, practice simple sketches to get the shape and proportions right.
Since I don't have access to the actual 'bbrae muscles fanfic', it's hard to say for certain. It could be a work of fanfiction where the writer has a particular fascination with the characters Bbrae and decides to explore their muscular build in the context of the story. Maybe it's a story set in a sports world where their muscles give them an edge, or it could be a more comical take on how their muscles get them into funny situations.
Well, without more context, it could be about a girl's journey in building muscles. Maybe it's her story of starting to exercise, facing challenges like strength training for the first time, and gradually seeing her muscles develop. It could also be about a girl who has natural muscularity and how she deals with society's views on it.
She might be initially stunned. After all, Percy is a hero and his well - defined muscles could be a sign of his strength and capabilities. It could make her blush a bit, secretly admiring his physical form while also feeling a bit flustered as she tries to maintain her usual composed self.
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.