Why is my Oncidium twinkle flower spike turning yellow?

[Wade Coleman]

I have an Oncidium twinkle fantasy that has been working on 4 flower spikes for what seems like an eternity. Slowly, but surely, they’ve been growing taller and branching out very nicely, but I began to notice that one of them has begun to turn a straw yellow color towards the upper half. The lower half is still green and branching nicely. What could be the cause of this? Should I cut the spike down to where it is green and let the lower half grow? In the photos you’ll see a picture of the questionable spike compared to a healthy looking spike.