good eye! i grew up loving that movie, so i think i put that plant in a lot of my designs. lol
as for the marker shades, i think that's the overlap of where the markers get put on the paper. if you go over the same area more than once, it makes it darker. i'm only using two grey markers here. i'll do the lighter area first, then go back over it with the darker one second.
i used a 6H lead for the sketch. a Sakura Micron 05 ink liner for the inked lines. a Tombo N15 Black brush pen for the larger inked areas. then its only two grey markers for the rest of the shading. a Copic T2 and T4