Curiosity piqued by my neocolor experiments in the moleskine, my daughters followed suit and tested neocolors with water. For this art project, we used heavy cardstock, Caran D'Ache Neocolor II wax crayons (these are water soluble), a paint brush and water.
My older daughter drew a vivid ribbon edged with white neocolors, then painted with a wet paint brush.
My younger daughter did some color mixing ~ blues and whites on black paper painted with water.
Each planet was painted with two colors and highlighted with a bit of water on the brush.
My daughters enjoyed this project; they were actually hesitant to put water on their art when it was time for that, so be sure to make your plan clear from the start. If the children decide not to embellish with water, perhaps they can draw another piece specifically for that purpose. It's their art, so no worries! My kids liked seeing the colors blend and get brighter. Variations include selectively wetting only certain parts of the painting, or drawing with neocolors on wet paper.