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I am familiar with the phrase in a [number] (seven, forty etc) when referring to sizes of clothes or shoes (while shopping).
Is it also okay to say in a/the ""? Cuz I often hear in "" (e.g. in red). But here's a dialog from Friends (The One Where Nana Dies Twice).

Aunt Lilian: Could we see something in a slimmer heel?
Ross: Okay, I have nothing in an evening shoe in the burgundy. I can show you something in a silver that may work.

Is the phrase 'in an evening shoe' okay?
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Yes, that sounds like normal shopping conversation.
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lagatawI am familiar with the phrase in a [number] (seven, forty etc) when referring to sizes of clothes or shoes (while shopping).Is it also okay to say in a/the ""? Cuz I often hear in "" (e.g. in red). But here's a dialog from Friends (The One Where Nana Dies Twice).Aunt Lilian: Could we see something in a slimmer heel?Ross: Okay, I have nothing in an evening shoe in the burgundy. I can show you something in a silver that may work.Is the phrase 'in an evening shoe' okay?
When the word or phrase is an adjective (color), we do not use an article (a/an/the).

When it is a noun phrase, use the article "a/an."

Okay, I have nothing in an evening shoe in the burgundy.
I can show you something in a silver that may work.
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Comments  
Maybe regional variation again.

If I have one pair of shoes that has a high heel in black and another by the same designer with a low heel in navy, it's very easy to me to imagine saying to the sales person "I likes these [holding out the high heels], but do they come in the navy?"
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