Well, if the leaves aren't looking good, then you want to trim that stalk back to the base of the plant.
It can be a couple of things causing the yellowing. It may not be getting enough water, it may be getting too much water, or the light may be wrong.
This type of orchid is hybridized from one which grows beneath a forest canopy, so it does not tolerate direct sunlight well. It likes morning sun, ie, eastern exposure, or indirect light. Bright, indirect light is best.
The roots, which are visible, should be growing and green. If they are black and wet, you are overwatering. If they are brown and crispy, underwatering. The plant should be watered approx. once a week.
I've attached a picture of healthy orchid roots. The material coating the growing root is called velamen, and it should be a little bit shiny, and the growth past there should be green. This is your goal as an orchid owner.
Sorry for the phallic nature of these images, but nature does tend to repeat itself.