Here is a 21-year-old version of the blonde beauty shooting a promo pic for "The Big Bang Theory" in Los Angeles back in 2007 (left). This was just the beginning of her 12-season run on the hit show, playing the role of Penny.

And, 16 years later the beautiful actress has diverted from her blonde days and recently freshened up -- going brunette -- at the premiere for Peacock's "Based On A True Story" in WeHo (right).

True story ... she's still a force to be reckoned with in Hollywood, but the question is ...