Article: The existence of literary and colloquial readings (文白異讀), called tha̍k-im (讀音), is a prominent feature of some Hokkien dialects and indeed in many Sinitic varieties in the south. The bulk of literary readings (文讀, bûn-tha̍k), based on pronunciations of the vernacular during the Tang Dynasty, are mainly used in formal phrases and written language (e.g. philosophical concepts, surnames, and some place names), while the colloquial (or vernacular) ones (白讀, pe̍h-tha̍k) are basically used in spoken language and vulgar phrases. Literary readings are more similar to the pronunciations of the Tang standard of Middle Chinese than their colloquial equivalents.

Now answer this question: colloquial reading are usually used where?
spoken language and vulgar phrases