Materials are simple, and inexpensive. A coffee filter, 9" x 12" piece of black construction paper, glue, newspapers for any mess, several sheets of white copy paper, and water soluable markers such as Crayola, spray bottle with water.
Color the filter with a variety of color, and since colors will flow and mix, don't overlap any. You especially want children to see the mixing come together spontaneously. Before spraying, set the colored filter on top of a piece of white copy paper.
The greatest problem with children is cutting beyond what will be the edges of the colored filter, so to prevent that from happening, use a crayon to trace the filter in the center of the construction paper.
Take the traced filter now, and fold...being careful that when folded the diameter/radius of the circle matches. That is...fold across the actual center of the circle.
I make a point to demonstrate cutting interesting "freeform" shapes, and remember to instruct them to cut along the folded edge, staying well within the traced circle. To create even greater interest, holes made with no connection to the folded edge should be made. Younger children will often poke and injure themselves trying this; however, if you put a stack of newspapers down and demonstrate pushing the point of a scissors down and twisting back and forth, such holes can be made safely Now we glue the cut out freeform, and emphasis is on using small amounts of glue. Then, turn the glued construction paper over and position it over the colored filter, lower and press down and smooth over with hand. tape to window and enjoy.