It might well be a color difference due to partial gel. I'm more inclined to think it's a color difference due to moisture content. The outer "rind" of a soap bar is usually drier than the center, and sometimes there's a color difference due to this difference in moisture.
The clues I use to figure out the difference --
A partial gel is usually an oval or circular shape and the partial gel ring is usually visible when the soap is first cut. It can lighten and become a little less obvious as the soap dries, but in my experience it remains visible even after a good cure.
A rind will have the same shape as the outer surfaces of the soap bar -- rectangular rind on a rectangular soap. I usually do not see a rind when I cut newly made soap -- it only appears after the soap has had a little time to dry. The rind will only then be visible on a freshly cut surface that exposes the higher moisture center to view. Eventually this rind disappears as the freshly cut surface dries out.
