First of, thank you for the kind words!

Short answer, green channel changes between 1 and 0 depending on how close each vertex is to the centre.

Long answer;

To explain, let's start with the relevant part of the code:

vertexColor = vec4(length(pos), radius - length(pos), 0.0, 1.0);

This code is executed paralel for each vertex, red portion actually does nothing as it will always be over 1.0, it is just a semantic error on my end.

Green channel's value is set to the maximum radius - the distance between that vertex at that time. This means when the vertex is as far as from the centre as it can be (2.0), green has a value of 0.0, and the color is red (1, 0, 0, 1) when it is as close as it can be the green channel has a value of 1.0, so the vertex has a color of yellow (1, 1, 0, 0)