You nailed it. People like OP, long time customer who wants to buy parts to see if they look better, should be the shop's bread and butter. The only thing I'd change is, "That will look unique" or even "awesome" - because this is purely a matter of personal taste. But egging him on, encouraging, enabling or whatever you want to call it, that's what the shop is there for.

I bought a black seat post for my road bike, because I noticed that every other bike in the house had a black one. I have no regret over that, an objective waste of money, yet had I gone into a bike shop for it and been told not to do it like OP I'm sure I'd have been put off. Actually I did. I went into several to browse what they had on display, but I didn't want whatever it is that made their posts so valuable; I just wanted black. I didn't ask and I don't blame the shops for not trying to sell me, but I was obviously bargain-bin shopping not like OP ready to order. Had I asked about one and been told that's a bad idea, I'd be irked.

In OP's case, at least be interested enough to find out exactly what OP was looking for in bars and stem beyond simply the color. I can understand OP's ire. You don't let a customer like that walk out the door without trying to sell him.