I'm using Blender 2.73 on MacOS.
I've been happily blending away for months since the last time I had a prob with modifiers. But not much issue lately. Suddenly I had a crash (I was subsurface-modifying and it went into a perpetual spin that wouldn't finish.)
I lost some work, but picked up from my last save again and kept going. As I came to the steps where I need a few boolean difference operation, I couldn't use it anymore - it's shaded red (see pic) and it failed to apply.
I added a cube and tried with that, and it failed to apply also.
I created a new empty file (below), and it also shows boolean as red, and when I try to apply, it does strange things. Intersect instead of difference etc.
Here's a copy of the file for download - just two cubes. Can you see some setting that's precluding me from using the boolean modifier or making it behave weirdly?
Question then is what does the red shading indicate? I searched the docs for "boolean modifier" and "red" and can't find any explanations. Googled wildly without any success.