Last year I did basically the same lesson with m 3rd grade students but had them use just two colors. It created kind of an Op Art effect but confused the majority of my 3rd graders. This year I wanted to keep some of the elements- overlapping to create positive and negative space and interesting shapes, and creating interesting compositions by filling the entire space.
I collected lots and lots of leaves to bring in for this project. I also had projects using leaves planned for two other grades at the same time so the leaves were used several times. I tried to find a variety of leaf shapes and sizes for the students to use. I think I had one student in almost every class ask if the leaves were poisonous.... yes, my secret plan is to poison my students with leaves in Art class. Anyway, the first class period was spent tracing leaves with pencil so that there was lots of overlapping and the space was filled. Some students had time to start tracing their pencil lines with sharpie in the first class, the others traced in the second class.
At the beginning of the second class, I showed the students how to trace the outline of the shape carefully before coloring in the middle. If the students actually use that tip, they do a much nicer job getting things filled in to be nice and solid. I showed examples from last year in our Artsonia gallery and told the students they could do the Op Art thing with two colors or use whatever colors they wanted- the only guideline with color was that no two shapes that touched could be the same color. I found that I had to reiterate again and again that everywhere the leaves overlapped created a new shape. Some still focused on each leaf as the shape and not the smaller sections. Only a few students wanted to try the two color route, most used several colors. I thought it was interesting to see the color combinations they came up with. Some planned it out ahead of time, some would just choose one color at a time and probably used every marker color available. Anyway, the projects were a big hit and got lots of positive comments when displayed in the hallway and I think I'll teach it again next year!