I think you're thinking of a pixel on a display? Because arguably that's just a rendering of a data set. The pixels the article are referring to are the data se

I think you're thinking of a pixel on a display? Because arguably that's just a rendering of a data set. The pixels the article are referring to are the data set, a set of point samples. In which case it has no shape - since it's a point - and the r, g and b components are just different samples at the same point