"In China the hibiscus is associated with wealth, glory or fame. All hibiscus flowers are short-lived, and so in many cultures they symbolise women and youthful or delicate beauty or perfection."
Tarn wanted Teh to colour in the hibiscus she drew in purple as it would have meant that Teh was paying attention to her (her chest, cough) like in previous episodes. She knows that Teh is acting off so she wanted to test if he'd colour it purple. But he in fact coloured it red, the colour Oh-Aew wears often and is associated with. It could also be because of the hammock scene where they put red hibiscuses is each others hair.