You can experiment with your roles if you are Kamal. Or say Vikram. But if you are a commercial hero, you have to stick to certain rules. That, however, is only a common notion. Ajith veered off the beaten path with his negative role in Mankatha and emerged super successful. He proved that a hero can have shades of grey (fifty and more indeed).
Our onscreen heroes have one thing in common – they are evergreen. The most important aspect of maintaining that tag is to have a pate of jet black hair, no matter how old. Ajith, however, did not let dye take his life (or hair) over. As a result, the salt & pepper hair is the trendy thing to have happened to a Tamil film hero. Mankatha, Arrambam, Veeram and looks like the salt & pepper trend is here to stay, thanks to Thala.