Well most white African species are white I believe in order to attract night time pollinators, moths primarily!
Cattleyas are not easy to repot and unifoliate are different then bifoliates. Take a bifoliate as an example. You MUST repot them when you just begin to see new root growth. If you wait until the new roots are an 1" or 11/2" long, you may have waited too long!! Why?
Because Cattleya roots are very EASY to damage. Once damaged, they typically stop growing or die back.
Just for the purpose of a discussion, let's say a new growth on a bifoliate Cattleya makes 5 new roots on every new growth, no more, no less. That growth rarely, if ever, produces more roots. Let's us say each root grows to 10" in length. That is 50" of roots, 5 X 10= 50". Then because it is repotted and the roots are too long, you break 2 off. They die back to nothing. 2 other roots reach 5" combined length and stop growing. But little old root #5 grows to 10", so instead of 50" of roots possible, you end up with only 15"! That is already just 30% of what you SHOULD of had.
This is why you set back bifoliate Cattleyas when you repot. Without that missing 65% of roots, the new growth is smaller, it is thinner, weaker and maybe blooms, maybe it doesn't, and you have one smaller flower instead of three larger flowers! WHY? Most probably root loss. The plant could not take up adequate water and fertilizer. It was handicapped by potting at the wrong time or potted incorrectly!
You CANT water more to make up for the difference because the remaining roots can only absorb so much. With more water, the plant doesn't dry out properly and more roots die. AND IF you do not correct your error, this snowballs into a weak, sickly Cattleya that finally gives up and dies! Now I have no real hard evidence that this is what happens but experience, logic and knowledge makes me think that this is exactly what happens!!
At least that is my messsge. I have heard 3-4 prominent orchid growers harp on roots, roots, roots and roots over the last 5 years. Maybe I am slowly reaching everyone?!?!?!