The crown (or at least the base of the leaves) can also have some measue of purple on them. I have a phal growing a new leaf, and while the light it gets is not enough to affect the old leaves, the base of new leaf is very tender tissue, and has gotten purple from the light.
Having yellow is not good though, you want everything to be green.
__________________ Camille
Completely orchid obsessed and loving every minute of it....