[semi-extrinsic]

So imagine you have three fluids that don't mix which are colored red, green, blue. You can make drops of these fluids on a piece of paper, and manipulate their shapes e.g. with a pipette.

This article is saying that mathematically, it's possible to make three single, continuous but weirdly shaped drops of these fluids, that together fill a square, in a way such that if you consider the three drop outlines as seen from above (e.g. by a camera), they all have the same outline.

[cousin_it]

Well, they all have the same boundary points. A boundary point is a limit of a point sequence lying inside the shape. But if we define a different concept of "outline point" as a limit of a path lying inside the shape, then I think the three shapes won't have the same outline.

[nerdponx]

The construction shown on the page makes me think of this as if the three fluids are infinitely mixed, if that makes any sense.