Acne pigmentation and dark spots for patchy skin (commonly in 30 ml serum or gel formulations) refers to targeted skincare treatments designed to fade post-inflammatory hyperpigmentation (PIH), stubborn dark patches (melasma), and uneven skin tone left behind by healed acne. These products typically utilize a combination of active brightening agents and exfoliating acids to reduce excess melanin production. 
Key Aspects of These Treatments:
What it Treats: Specifically targets flat, non-textured dark spots, post-acne marks, sunspots, and patchy skin tone.
Key Ingredients: Common, effective ingredients found in these 30ml serums include Alpha Arbutin (inhibits melanin), Tranexamic Acid (targets inflammation-linked pigmentation), Kojic Acid (derived from rice/mushrooms to brighten), Niacinamide (Vitamin B3 to fade marks), and Vitamin C.
How it Works: These serums use a “deep penetration formula” to target pigmentation at the source rather than just on the surface. They reduce the excess melanin produced by skin cells during the inflammation of a breakout.
Targeted Skin Issues: Patchy skin (melasma), dark acne scars, sun-induced tanning, and uneven complexion.
Application: Usually applied as 3-5 drops on cleansed skin, twice daily (AM & PM), followed by sunscreen during the day. 
Common 30 ml Products & Actives:
Alpha Arbutin 2%: Focuses on fading dark spots and blemishes (e.g., Minimalist, The Ordinary).
Kojic Acid Serums: Often paired with alpha arbutin for quick fading of stubborn marks (e.g., The Derma Co).
Tranexamic Acid Serum: Targets melasma and persistent pigmentation.
10% Niacinamide: Focuses on fading marks and reducing sebum.