Safety Stop:My question was not only about color but type of regulator too. If we should have the same type/model of regulator instead of an octopus (never as good as the regulator the diver used).
In DIR we use the terms "the primary" and "the backup". We don't use the terms "octopus", "secondary", "Safe Second" etc...
The only criteria for each regulator is that it is a "quality" regulator and the 2nd stage not be of an upstream design like the Poseidon Jetstream. Knowing that then one could use the same type/model for both primary and backup. However it is suggested that the backup be of a non-balanced design (ie: a Scubapro R190 instead of a G250), but most people seem to just use a detuned version of whatever they use as a primary.