The subtle reason why Penny wears the red Leopard print blouse:
When Sheldon spoils Leonard’s Harry Potter reading experience, Leonard gets really angry, argues with Sheldon and finally flees to Penny’s apartment. Over there, he continuous to moan about how terrible Sheldon is, while Penny (Kaley Cuoco) cleans the dishes and makes cynical comments. The situation culminates when Leonard says that he wants to move in with her, which makes her uncomfortable and she tries to sneak out of the trap.
Picture Source: Variety Copyrights by CBS
In this scene of The Spoiler Alert Segmentation (S06E15), Penny wears a Rhubarb red and black leopard print blouse by fashion brand Equipment, which visualizes perfectly her situation that she feels trapped like a wild animal who Leonard wants to domesticate by moving in her private sphere. Penny is in a sort of panic, because she loves Leonard, but doesn’t want to live with him permanently. She is too nice to tell this to him directly and therefore she tries unsuccessfully to sneak out of this situation, like a wild Leopard who smells trouble instinctively. Her Emo like style, with a black skirt and makeup might be another hint that she feels quite desperate in this situation.
I don’t know if costume designer Mary T. Quigley had all of this in mind, when choosing this shirt for Penny in this scene as her ‘equipment’ against Leonard, but I think it’s a reasonable assumption.