Some use brown or black. I personally think that Y should be changed to B so you can offically use blue for cool which makes more sense.
"I could have ended the war in a month. I could have made North Vietnam look like a mud puddle." "I have little interest in streamlining government or in making it more efficient, for I mean to reduce its size. I do not undertake to promote welfare, for I propose to extend freedom. My aim is not to pass laws, but to repeal them." Barry Goldwater
ON Rheem/RUUD, the blue wire is used for the reversing valve (which BTW energizes to heat). On Trane and most other brands the Orange wire is used to energize the reversing valve to cool.
So what does this have to do with the common terminal of low voltage control circuits in residential and small commercial?
Since Rheem/RUUD use blue for the reversing valve, then a different color needs to be the common. Usually one finds the brown wire used for commn in this brand. Sometimes folks will use black.
On Trane, the blue wire is used for common most of the time. I have seen black used for common, but never brown (brown is sometimes used for other functions on Trane, but they slip my mind at the moment).
Here is a question for you: If you were installing a 2 stage furnace with two stage A/C, which colors would you use for second stage heat and second stage cool???
Reminds me of a changeout where the lead installer was having a heck of a time wiring a Trane outdoor unit to two twinned furnaces inside. He kept calling me on the radio and I was trying my best to help him without any success, so I finally went over there. I checked his wires at the furnaces and at the outdoor unit and it should have worked, until I started checking voltages and got all kinds of funny readings. Then we started tracing the wiring under the home and found that it had been spliced 4 times and the colors changed at each splice. What fun....
For Y2 I usually use pink and for W2 I usually use black.
At 1220 L Street NW, had an Electrician install and wire up 460 x 24 VAC transformers on a Liebert units for the water detection cables. Luckly he left the fuses out, as he wired the 24VAC side to the 460 and the 460VAC side to the 24VAC detection circuit.