If you see changes in color, you want to either harvest it now. Or watch it closely everyday with coffee filter paper or whatever else to catch any seeds in case the capsule pops open while you're not there.
I allowed the capsule to go at least 9-10months before harvesting but one particular capsule did pop open and I learned it the hard way. The changes in color on the capsule happened so fast (in just a couple of days) and before I realized what was going on, it popped open and I lost about half the seeds. I was able to save most, though.
Ray- Unlike some other orchids whose ovary expands to a considerably large size compared to its original size, most of the Paphiopedilums I dealt with changed very little in their capsule size. Perhaps, just a little bit of swelling but not much. One particular pod did swell up quite a bit more than others, but still it is pretty much nothing compared to things like say, Phalaenopsis capsule. Majority had high germination rate regardless.