Paint mini canvases, put together in square, use squeeze bottle to swirl white paint over all of them. (middle three diagonal = same color; two top diagonal=same color; two bottom diagonal=same color; top corner on color, bottom corner one color)
Watercolour triangles let students paint paper with watercolors, wet on wet, similar colors. Let dry and cut into triangles. Students pick triangles and glue down randomly, leaving white around each one. Maybe try with black or other color background?