The update to WW Pro texture exporting fixed some issues form last week but introduced another that can throw off gamma or alpha. So there will be another fix probably today.
I have not had a chance to fully test this, but I just loaded a new DEV version of WW (http://wallworm.com/projects/utilities/dev_wallworm_model_tools.zip) (that can be installed over WW Pro as long as the WW Pro install is within the last month or two). So if you need to fix this bug ASAP and are fine to test, feel free to load the DEV version. Let me know if it fixes any texture export problems.
This is now fixed in WW Pro 2.708.