I find it interesting how much texture and shape a repetitive group of lines can create! The Zentangle uses three tangles that use lines in various ways.
No, it is not your imagination. You really are seeing a small bit of blue and red here. I used a bit of colored, pastel pencil along with the graphite shading in Isochor (red) and Olb (blue). I just wanted to experiment to see what would happend if I tried it.
Zentangle drawn on Strathmore Vellum Bristol using a black, Micron pen. Shading done with graphite and pastel pencils.