This is just a project to explore using transparent pen. R, G, and B are the color values that the pen will have. Think of them as the ingredients into the final color that is used to draw. Each ranges from 0 (none of that color) to 255 (a lot of that color). So, if you set R to 255 and G and B to 0, then the star will be drawn in bright red. A is the Alpha value, that determines how "see through" the color is. 255 means that the color is totally opaque and you won't see anything through that color. 1 means almost completely transparent, and you will see a lot of background through the color. 128 is somewhere in between between, you'll see about 1/2 of the new color mixed with whatever was already on the canvas.
Thanks to @mbaruah https://scratch.mit.edu/projects/90843971/ for a nice demo project with random transparent pen colors. https://wiki.scratch.mit.edu/wiki/Pen#Transparent_Pen has the official description of transparent pen.
