the barycentric issue is https://github.com/observablehq/plot/issues/2312

Correction about "barycentric" interpolation: I think colors are considered 'categorical' values so the random selection is used instead of interpolation.

That's if you pass the colors as CSS4 such as `rgb(x y z)`; this is not supported yet in Plot (but see https://github.com/observablehq/plot/pull/2145 for a fix).

Using rgb(1, 2, 3) doesn't work at all at the moment, I've already changed the CSS colours to hex notation.