Welcome to Orchid Board
Purple hue relates mostly to sunlight, but I have noticed that plants that are limp tend to turn purple in the sun easier than those which are healthy. Some can take going quite purple and still be healthy, it all depends on the ancestry of the plant. Those with dark colour flowers tend to be more purple than those with lighter flowers.
If it is going limp that is more of a concern. You say it has gone a different colour in the sun... what colour has it gone. If it's gone pale and yellow/brown then it could be actually burnt and you need to reduce the light levels you are giving it (some phals can cope with more light than others).
Limpness tends to indicate a root problem unless the sun has completely burnt that leaf. It's best to take a look at the roots if there is a possibility of a root problem. Post photos here so we can try and advise on the roots.
Oh, and like Philip I do advise reading that link he gave you all the way through as it has some really good information about Phals and how to avoid killing them.