To quickly answer your question, there is no etiquette.
However unspoken tradition is to have standard octos as yellow with a yellow hose.
On the other hand, Bungee backups are usually both "primary" second stages by tradition. Color coding never really applied to this config.
This is more to the fact of the original concept of the configurations, rather than color coding etiquette.
- If you're using a bungee backup you'll want both second stages to be relatively high performance. You're primary is what you'll be using until an OOA incident in which case you'll want your back up to be just as good since you'll be switching to that.
- With standards octo configs this thought never really took hold since you rarely use it and in the case of an OOA situation, you're still not using it, unless you were trained to give your primary away.
If you decide to make the switch on your second stages then you'll have to cover this personal configuration with your dive buddy during the predive planning. And if another diver comes up to you OOA you should be prepared to take control and hand off an air source, rather than "exposing your hip" (which I would never recommend; passive rescues are just silly).
IMHO I think it's a lot easier to spot a colored reg rather than tracing a colored hose to the second stage. But don't let this single reason stop you, chances are you're primary will be taken away first if an OOA diver gets the jump on you. In which case you'll be glad to have a higher performance "octo".