While out suit shopping, a clerk helped us choose ties to match our suits. He provided excellent colour combination suggestions. I complimented him and asked him what his approach was. He told me he was simply choosing complementary colours, like on the colour wheel. It's important for his line of work (selling suits) to have expertise in at least basic colour theory in order to create objectively good colour combinations on the fly.

How about you? In your line of work, what do you think is important to know?