Neither mdbg.net nor Yellowpage nor Pleco seem to know this word, though it seems to be common. It stumbled over it in the first HSK4 mock test:
这个节目我一直在看,它介绍了很多生活中的小知识,包括怎样选择牙 膏,擦脸应该用什么毛巾,怎样远离皮肤病等等。很多以前我没有注意到的 问题,现在通过它了解了不少。
I thought it must be to clean one's face or something similar. A web result suggests you can use Vaseline for it, so it's focus might be more on grooming rather than cleaning?
Anyway, what does this mean and why don't dictionaries know that? Is it very colloquial?
Christopher's right that the base meaning of 擦 is "wipe." I hear (and say) it most often in the phrase 擦黑板 ("erase the blackboard").
I would just add that because the meaning of 擦 is pretty general, the correct interpretation of 擦脸 will depend on context. It might mean “wipe your face with a handkerchief", "dry your face with a towel", "wash your face", etc. In your example sentence, I would actually be inclined to interpret it to mean "wash your face," but it's not super clear.
Just to give you an additional datapoint, here's a definition and example sentence from Baidu dictionary (emphasis mine):
用毛巾等揩脸；洗脸。 陈白尘 《无声的旅行》:“我们在五分钟之内完成了起床、穿衣、擦脸等一连串动作。”
In this case the context makes it clear that the meaning is "wash your face" to go along with the rest of the morning routine of getting up and getting dressed.
擦脸 is not a single word but a verb (擦 cā: wipe) plus a noun (脸 liǎn: face) that serves as an object. "擦脸应该用什么毛巾" means "what kind of towel you should use to wipe your face [dry]".
在汉语的日常使用中. 擦脸表示 脸上有汗 或者水 或者其他东西. 需要擦掉. 早上起床之后,应该说:们在五分钟之内完成了起床、穿衣、洗脸等一连串动作。”
