White to yellow, never heard of this before, not sure what it could be but yep a would help. I have seen and known of white fly on hibiscus...the orchid your referring to is it outdoors or indoor enviroment? please!!!
I have seen in a paph book writen by Lance Birk a pic of a paph with white leaves. In that particular case, it was due to a disease called Rhizoctonia solani. If you take the plant out of the pot and look at the roots and there is evendence of 'webbing' right on the roots, this could be the reason-this disease. But not sure.