Leaf work is great - but best to do it with fresh leaves, in late spring or summer. A revisit of the fern leaves, with a different threesome of colors.
Color play: who says a triad always has to be the primary colors? It can be different colors on the color wheel, that will give you a nice range of mix colors. Like here.
Image is 20 by 25 cm (approximately) - as the plate is that size (8 by 10 inches).