It helps to separate two things. Flat brown, red or purple marks left after a spot, called post-inflammatory pigmentation or erythema, are not true scars and usually fade gradually over weeks to months, though darker skin tones may take longer. Daily broad-spectrum sunscreen is important, because sun exposure darkens and prolongs these marks.
True acne scars involve changes in skin texture, such as pits or raised areas, because the skin healed with altered collagen. These are more stubborn and generally do not fully disappear on their own. The best results usually come from preventing new acne and treating active breakouts early to limit further scarring.
For established scars, professional options exist and are best discussed with a doctor or dermatologist who can match the treatment to your scar type and skin tone. Be cautious with strong over-the-counter or online products that promise fast results, as some can irritate or worsen pigmentation. This is general information, not a personalised plan.