In South Africa the colours are (or at least were whhen I left 6 years ago) Red-Whit-Blue. The White would fade in the sun and become yellow. Here in the UK (before we moved to the mad colours of now), the Yellow would fade and look like dirty white. Looks like a no-win situation!
However, that is off the subject! The rule I remember was each and every wire in a cabinet must be definable by its colour as to its use e.g. all white wires were control, all red were power, etc. If you take this across to your situation I believe, seeing as white is already defined as control, yellow should continue as your phase colour.
But please don't take this as law (simply because my experience is law diametrically opposes common sense!).
M.
