着色 is a verb-object separable compound (离合词) meaning 'to apply color, to tint, to colorize'. The first character 着 (zhuó) means 'to attach/to apply' (distinct from the homograph zháo in 着急), and 色 means 'color'. As a separable verb, elements like aspect markers (了, 过) or duration can be inserted between the syllables, e.g. 着了色 ('has been colored'). You commonly use it with 给 to introduce the object: 给画着色 ('color a painting'). This word appears in contexts ranging from painting and fabric dyeing to digital photo editing.