Trees in gradation

In  classes of  7th Grade (12 year-old) we have painted a forest of trees without preliminary drawing, using directly the brushes and the acrylics paints on the white paper. To make the effect of the depth we painted different color planes in tonal gradation. Most of the students chose to use black and white, while someone has used a color instead of black. We talked about the concept of MONOCHROME in art, such as a painting, a drawing or a picture, made ​​with a single range of colors in tonal gradation.