What exactly does the XOR option do? I copied a large red square with a slight gradience to it and made one of their ink effects "XOR" and when placing it over the other it made some funky patterns! Well, weird circles!
Well, it stands for "Exclusive OR". Logically, it takes the colors (its color and the one it overlaps) and compares the bits. The resulting bit is deemed TRUE if either one OR the other bits is TRUE, but not if both are. This varies from a normal OR, which, if both bits were TRUE, would make the resulting bit TRUE.
Like this:
0 XOR 0 = 0
0 XOR 1 = 1
1 XOR 0 = 1
1 XOR 1 = 0
...and that's what goes on in the equation.
Effectively, though, it inverts the color underneath over the color on top. So if you had a solid white object with the XOR ink effect on it, everything underneath it would turn the opposite color.