Append channels in math evaluator

Let suppose I have a expr() of one channel within the math evaluator for use for draw(). I want to append the expr() vector with color vector as if I were appending channel. What should I do?

For more context, I was thinking of having a set of extracted x,y and insert transparent colored star into a new image via parallel processing. A much faster version of @samj filter where the image turns into several lines around a point at the edge detected areas. Theoretically, this enables more option and speed.

By the way, I’ll provide samples soon.

Like this? (Swap R and B.)

gmic sp tiger f [i2,i1,i0]