My phalaenopsis has been looking a bit sad lately, and it's been about a
year since he was last potted, so I figured it was about time.

Two somewhat disturbing things. First of all, there wasn't much root growth.
Barely enough, in fact, to compensate for the old, dead roots. Secondly,
much of the new root growth was transluscent yellow, rather than the pale
green I'm used to. It looked almost like on the verge of rotting, but they
certainly seemed firm enough.

Does anyone know what I'm doing wrong? Underfeeding (possibly)? Overwatering
(unlikely)? Something horrible under the stairs?

Thanks,
Colin