GIMP blending modes

I keep having difficulty with the Value blending mode on GIMP. Every time I want to use it, it's greyed out. How do I get it to work?

http://forums.eyesonff.com/showthread.php?t=71210

*cough*

Quote:

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Originally Posted by starseeker

I keep having difficulty with the Value blending mode on GIMP. Every time I want to use it, it's greyed out. How do I get it to work?

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are you making a selection that has multiple colors all on a single layer?

Yes, but it worked before.