There could be multiple factors. His role models, whether they were his teachers, older siblings, or people he read about in books, might have inspired him. Also, the realization that hard work brings rewards, like good grades, praise from others, and a sense of accomplishment, would have made him keep up his hardworking attitude.
Rohan was a boy who always put in great effort in everything he did. He woke up early every morning to study before school. He was dedicated to his schoolwork, always completing his assignments on time and with high quality. He also helped his parents around the house, doing chores without being asked. His hard work paid off as he got excellent grades and was respected by his teachers and peers.
One common theme is the exploration of their complex relationship. Given their different personalities in the original series, fanfictions often delve into how they interact and come to understand each other better. For example, Josuke's more easy - going nature contrasted with Rohan's often - prickly demeanor.
Fanfiction.net is also a popular place. There are many fanfic writers who contribute to the Josuke x Rohan category there. However, the quality may vary, so you might need to read a few to find the really good ones. Some might be more focused on their friendship, while others could explore a romantic relationship between them.
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.