Growing concerns regarding health and eco-safety have led to a resurgence in the recognition of natural products. Although plant colorants are often mentioned in ethnobotanical studies in northern Thailand, they have not been comprehensively examined. This study aims to (i) investigate the colorant plants used by ethnic minorities in northern Thailand and (ii) compare colorant plants used among the groups in northern Thailand with other regions worldwide. Data on colorant plants used by ten ethnic groups across 142 villages in northern Thailand were extracted in a systematic review of 42 published references. The importance of the colorant plant species was evaluated using the ethnobotanical index “use value” (UV), and the homogeneity of knowledge was assessed using the “information consensus factor” (ICF). The similarity of colorant plants used among the ethnic minority groups and beyond was evaluated using Jaccard’s index. The 42 published references included information about 104 colorant plant species belonging to 85 genera and 43 families from seven provinces in northern Thailand. This represents nearly half of the 212 colorant plant species reported across all 72 provinces in Thailand. The most important species were Strobilanthes cusia and Morinda angustifolia, which are sources of blue and red colors. Fabaceae, Rubiaceae, and Lamiaceae were the plant families with most species used as colorants. The colorant plant species yielded ten different color shades, with blue being the most prevalent. The Jaccard’s index varied from 0 to 0.27 among the northern Thai ethnic minorities and from 0 to 0.13 for regions outside of northern Thailand. The regions located closest to northern Thailand exhibited the highest degree of similarity. This study provides valuable insights into the traditional knowledge and usage of dye plants in northern Thailand, contributing to preserving the cultural heritage and providing basic knowledge for the sustainable use of natural colorants for modern applications.

Historically, natural pigments have been used to color textiles, food, and cosmetics, but the rise of synthetic dyes in the 19th century reduced their use. Recently, interest in plant-based pigments has surged due to health and environmental concerns. Among Thailand’s ethnic groups, the Karen use plant colorants extensively, but their practices remain understudied. In this study, we document the knowledge of plant colorants of the Karen community in Chiang Mai province, assess the color stability of the plant colorants, and evaluate their cytotoxicity in HepG2 cells. Interviews with 113 informants revealed 52 plant species used for dyeing, primarily through hot water extraction. The most common color was brown, and Strobilanthes cusia had the highest color use value (0.93). The study identified 10 color shades, with blue showing the highest consensus. Color stability was tested at room and elevated temperatures, which showed that colorants from Oroxylum indicum were the most stable, and those of Strobilanthes cusia had the lowest toxicity (CC₅₀ = 994.1 µg/mL), while Artocarpus lacucha had the highest toxicity (CC₅₀ = 63.96 µg/mL). Oroxylum indicum, which has excellent color stability and moderate cytotoxicity (CC₅₀ = 294.4 µg/mL), is recommended as a promising natural colorant. This study provides valuable insights into preserving traditional knowledge in Karen communities.