This looks like phosphorus deficiency. When was the last time you added fertilizer, how much and what formulation? Possibly magnesium deficiency. Did this discoloration start at the margins to spread towards the mid vein? Best to know what you have added to the potting soil of your orchid. Is this primarily found on the lower or older leaves? When there is a shortage of a mineral/chemical the plant needs to photosynthesize and survive the plant will always send to the young growth at the top as the best survival mechanism. Phosphorus as are all the other chemicals (nutrients) plants need to photosynthesize when really low or insufficient actually promotes the making of anthocyanin (sp?) thus the purplish color. Your flowers will be wimpy and thin. ATP is energy for us for plants. For most of life. When ATP is broken down to ADP, Adenosein triphosphate to Adenosein diphosphate that little chemical change makes energy. See that last part with the phosphate? That is phosphorous. Lacking this P will cause a plant to slow way down. If you have fertilized, one thing that would cause this plant to not be able to take up the available phosphorous is the temperature of the soil. If it is cold and phosphorus is in the soil the roots will be unable to use this chemical. Fertilizer is needed in SMALL amounts. Excessive fertilizer is actually worse than too little.