A Plant that normally grows green leaves can have a condition called chlorosis which causes a severe lack in chlorophyl production. Chlorophyl is the green plant-pigment responsible for photosynthesis - the process which captures energy from light. Chlorosis directly impacts the fitness of the plant as the metabolism grinds to a stop, leading to death if untreathed. Leaves turn yellow-white. There are several causes for chlorosis:

• poor drainage leads to waterlogged soils, especially when it contains large amounts of organic material (peat, composted soil, sived leafmould)
• damaged roots: dig up an affected plant to check the roots. Nematodes and insect larvae are known to like orchid roots. Bacterial and fungal infections show up as unhealthy dark spots - often with soft tissue.
• compacted roots is often accompanied by bad watering techniques or fine soil. Immediately repot in soil with high drainage and low compaction (coarse grit/perlite/seramis)
• high alkalinity: too much calcium can cause a high pH, which makes it possible for other elements to interfere with the absorption of iron, rather than to a lack of iron in the soil. This occurs in neutral to alkaline soils when the pH is above 6.5
• nutrient deficiencies in the soil
  • lack of Iron: starts on the younger or terminal leaves and later works inward to the older leaves. Iron becomes more insoluble as the soil pH climbs above 6.5 to 6.7 (7.0 is neutral - below 7.0, the pH is acidic; above 7.0, the pH is alkaline). With most plants, iron can only be absorbed as a free ion (Fe++) when the pH is between 5.0 and 6.5. Other elements such as calcium, zinc, manganese, phosphorus, or copper in high amounts in the soil can tie up iron so that it is unavailable to the plant. Potassium is used to mobilise iron and a low concentration can stop iron-uptake.
  • manganese and zinc deficiencies develop on the inner or the older leaves first and then progress outward