Okay, this is pretty technical, so might be complete greek to some, but hopefully there's a photoshop expert or two who might be able to help me...
I have a project. I'm doing two separate files with front and back views of a group of characters. They're pencil lines, with two layers of the same pencils, one set to multiply and one to linear burn (linear burn picks up the colors behind it so that the final image, with both pencil line files, has more vibrant pencil lines instead of totally gray). I have a flat color layer underneath it, and on top of that I have a layer of a flat purpley-gray color which I erase highlight areas from, thus creating shading. I then set that layer to a blending mode, usually multiply or linear burn, whatever looks right for the lighting and colors. I scrolled through the modes and Difference looked awesome, exactly perfect, precisely what I wanted. Difference is usually a useless belnding mode to me, but for some reason in this file it was doing something I'd never seen it do before and it was awesome. So I used it, painted details on top of everything, then did a brightness/contrast adjustment layer to boost it all a bit.
Then I started coloring the back view file. Same color mode (CMYK), dpi (600), flat colors, shading colors, everything. But for some reason the Difference blending mode on the shading layer looks completely different. I've used the same colors in the flats, eyedropped from the other file with all other layers turned off to make sure it was the true colors and not the affected colors. I double checked the color mode. I double checked the color of the shading (again eyedropped with all blending modes and other layers turned off and opacity set to 100%). And the Difference blending more just looks like crap. Looks like Difference mode always looks. Looks right in all the light values - the dark values are where it's completely wrong. Light gray is turned black while dark gray has become lighter than the light gray, while in the other file the dark gray was what turned black and the light gray was just made a darker, cooler gray.
So what the hell, I know it can't just be a fluke, but I can't find what is different about the two files at all. Have any of you ever had anything even remotely like this happen to you, and could possibly help locate the issue? Maybe come up with something I may not have checked yet?