Hi Will,

I keep mine in shade/dappled sunlight. A few of mine get morning sun, which is ok. It is not as hot and vicious as afternoon sun. Heat alone will not hurt Nun's orchids. Yours looks like it may have been sunburn from direct sun. The sunburned leaf will not repair itself, but if you move it to shade, new leaves will be ok. Like you have been, keep it moist and move it to shade or speckled/dappled sun/shade.

Here are my Nun's orchid links. To quickly and easily open a link, triple-click on the link, right-click and select "Open Link in New Tab". I hope these are helpful - lots of good info:

http://forums.gardenweb.com/forums/load/lagard/msg0809270532382.html?13

http://homeguides.sfgate.com/propagate-nuns-orchids-62623.html

http://thedailysouth.southernliving.com/2009/03/04/hey-sister-get/

http://www.clanorchids.com/culture/phaicult.htm

http://solutionsforyourlife.ufl.edu/hot_topics/lawn_and_garden/nuns_orchid.html

http://anpsa.org.au/p-tan.html

http://herselfshoustongarden.com/2009/04/nun-orchid-phaius-tankervilliae.html

http://easttexasgardening.tamu.edu/tips/house/orchids.html

http://www.rv-orchidworks.com/orchidtalk/orchids-other-genera-bloom/13953-phaius-tankerville-nuns-orchid.html

http://www.orchidgeeks.com/forum/orchid-flasking-and-seed-germination/6047-propagation-of-phaius-tankervillae-nuns-orchid.html

http://orchidsamore.com/tutorials/Phiaus.htm

Happy gardening!
Carol in Jacksonville

The leaf color is the color that it is supposed to be a greenish yellow, the brown tip is either from too much fertilizer or from drying out, I would guess the latter.
Phaius (Nun's Orchid) like these are naturally found in bogs and marshy areas, they like their feet wet and I grow mine in a pot that is sitting in a saucer filled with water. The plants get huge and bloom with many spikes per pseudobulb

They're native here and they're called Swamp Orchids. Usually found growing in debris (vegetation) washed up by flooding.