The omittance of the possessive de can be made possible by many factors. In general, the adjective-noun phrase without de (e.g. hóng-huā ‘red flower’) is more likely to denote an integral category of red flowers, where the redness is the essential category of those flowers, whereas adjective-de-noun phrase (e.g. hóng-de-huā ‘flower that is red’) is more likely to refer to flowers that happen to be red (Li & Thompson, 1989, pp. 118-119).
Another factor that enables the omittance of de is the literacy of the modified noun. The more literate the modified noun is, the more likely thatdeis omitted. Thus, zhòngyào-de-rén ‘an important person’ cannot omit its de, whereas the synonymous zhòngyào-rénwù can go without de since rénwù ‘personage’ is a literary term. (Li & Thompson, 1989, pp. 122-123).
Li, C. N., & Thompson, S. A. (1989). Mandarin Chinese: A functional reference grammar. Univ of California Press.