Yeah sorry, I toss that term around rather lightly, like everyone knows it.
OPCF is a smaller mod I did for SH4&5; stands for Observation Periscope Color Filter. It adds a colored layer to the mask, can help a bit in low-light conditions.
The idea is historically accurate, though the effect on gameplay isn't nescesarily.
The differences between light and dark versions should be obvious, as well as difference between with and without OPCF:
http://www.subsim.com/radioroom/show...80&postcount=2