Pre or Post heresy?
In the old Index Astartes article they are shown in white with a red pad. However, more recently GW's painted examples seem to indicate they wear predominantly standard armour with some white heraldry - Corbulo wears red, and the Honour Guard example in the codex has the Sanguinary Initiate wearing gold like his squadmates. Even in earlier examples they do not appear to have all white armour:
Of course, Honour Guard members are initiates rather than full priests, and certainly the status of the Sanguinary Priests in the Blood Angels (equal to chaplains) suggests they should have their own heraldry.
I'd be inclined to say that if you have a strong preference, go with that, whether it is white or red. Still, I'd say initiates are more likely to wear the same colour as the rest of the squad tehy are in - its only full Priests that are likely to be in white, Corbulo not withstanding.