I have black and grey couplers, one says "in" and the other says "out", but they look exactly the same to me... Does it matter if I use them indiscriminately for gas in/beer out lines?
They are slightly differently sized. In particular, the grey gas fitting will fit over the "out" post, but the black liquid fitting will not fit over the "in" post without significant pain.
But, yes you should treat them as the separate things they are.