Short answer: best to avoid.

• What is ekashana: taking food only once in the day, with restraint from any extra “taste-taking” (ras‑sevan) outside that one meal.
• Lipstick is alankar (adornment) and usually a ras‑dravya. It can be tasted unintentionally when licking the lips, and many lipsticks contain himsak ingredients (like beeswax, shellac/lac, carmine). So, applying lipstick goes against the spirit of ekashana.

Tradition notes:

• Digambar: on vrata days, lepa–gandha–alankar (cosmetics, fragrances, adornment) are generally renounced. So lipstick should not be used.
• Shwetambar: during ekashana and paryushan-type vows, ras‑tyag outside the meal is observed; colored or flavored lip products are discouraged. If there is a genuine medical need, a tiny amount of plain, colorless, unscented, unflavored, vegan balm may be used as aushadhi (medicine), taking care not to lick the lips.

Best practice:

• Do not use lipstick on an ekashana day.
• If medically necessary for cracked lips: use minimal, colorless, unscented, unflavored, vegan balm only as medicine, and avoid licking. For stricter vows like upvas/proshadh, avoid even this unless essential.