These images are single brushstrokes on paper, photographed and manipulated in Photoshop (mainly with a motion blur and sometimes layered with a 20-30% opacity blurred version (set to the lighter color blend mode) to add some glow. I started with black acrylic on white and tried white acrylic on black using various brush widths (see below). Fun and simple to create.
Black acrylic on cotton rag (left) photographed and inverted in Photoshop (right)
White acrylic on black paper (left). Various brush widths and textures give different results (right)