Answer: Older leaves tend to lose their color just before they die off. It's nothing to be concerned about. In fact, if you break off the affected leaves, new leaves will grow in their places. The other two things that I am aware of that can drain foliage of color are salt toxicity, and being too root bound. Salt toxicity in this case would be from a build up of too much fertilizer in the soil - flush the soil, boxes or pots well with water a few times, and stop any liquid feeding.
I have also seen plants that are very very root bound develop white foliage - the roots are so thick and crowded in the bottom of the pot that there is not any room for soil, so the water and nutrients pass right past the roots and out the bottom. If you think that this is the case, you can either try repotting, or if that is impossible, try top-dressing plants with earthworm castings and watering with a weak solution of fish-seaweed emulsion and see if this helps.
It is possible, of course, that a combination of these two problems exist, but looking on the bright side, at least it's not mildew, which is harder to remedy.
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