Koolen made sketches and drawings in black and white or pastel shades, expressing the beauty and serenity of women with a calm and tender look, depicted in semi profile. After his death, all these drawings - very sketchy to delicately detailed drawings - are found in a large folder.
Girl with flowers, 45 x 60 cm, charcoal and brown chalk on paper
Sketch of a woman, 50 x 60 cm, charcoal on paper
Returning home, 1947, mixed media
Sketch of a woman, 50 x 60 cm, sepia conte crayon, blue, yellow and orange chalk