Based on the reviews, the NOSCA9 TROUBLE SERUM is primarily praised for its effectiveness in calming blemishes and reducing redness, rather than specifically improving skin tone. While some users have mentioned that their skin looks more settled and their complexion has improved, these comments are more about the reduction of blemishes and redness rather than a direct improvement in skin tone. Therefore, the primary benefit seems to be in addressing specific skin concerns like blemishes and redness, rather than an overall improvement in skin tone.