ISH - ethanol dehydration of whole mount embryos - Bleaching of signal (May/17/2011 )

Hi,
Can someone please tell me why NBT/BCIP signal bleaches out/vanishes after I dehydrate my embryos using ethanol?
How can I prevent this? Is dehydration even necessary?
Grateful for any advice on this.
Best,

the insoluble, colored, deposit formed when detecting with nbt/bcip is soluble in alcohol.

Yes, I know that some component of the dye is solubilized and the colour changes from purple to blue, but ALL colour shouldn't vanish, especially since usually I watch the colour develop for over 5 hours after adding nct/bcip.