Question
Wayne, I have several phals; one of them has a leave which I noticed is discolored, it also has a black spot on it. The other leaves on the plant look great.

I have moved the plant into my garage for the winter, I live in Orlando, Florida.

I noticed today that the sun shines on it through the garage door small windows in the morning; there are other Phals which are on the same bench, but those are OK.

I recently applied a systematic insectide per your recommendation on another question I had asked.

You can take a look at the issue by hitting the following URLs,

http://www.caripito.com/images/orchids/Phal5.jpg
http://www.caripito.com/images/orchids/Phal6.jpg
http://www.caripito.com/images/orchids/Phal7.jpg

Thanks.
Angelo

Answer
Angelo, the pictures are a little fuzzy but I note an area on the leaf which has been eroded away and is surrounded by the lighter leaf coloration.  It may be that eroded area was damaged by sunlight focused on that spot.  My best guess is that the damage provided an entry point for fungal attack which resulted in damage to the leaf chlorophyll. In the event this was the case, I would cut the leaf off about an inch below any yellowing and seal the cut with powdered sulphur, roottone, or other powdered fungicide.