I think the fins are all the same color, on both sides and their edges. Numbering them clockwise from back/left, fin #2 is in strong light coming from the right. Fin #3, facing directly toward the camera, is edge-on to the key light source, so its surface looks darker. Fins #1 and #4 look very dark because we're seeing their shadowed sides facing almost directly away from the key light. Fin #5 is mostly shadowed by fin #4, so its tone is mostly similar to that of #3, but the areas that are strongly lit look about the same as fin #2. Shading and highlights of the edges of fins # 2, 4 and 5 are also consistent with the directionality of the key light. The only apparent inconsistency to me is that there's no highlight visible on the edge of fin #1. But, it might be completely shadowed by fin #2.