Give your art a dark theme by using gloomy, maudlin colors. Blacks, grays and sickly browns express negative emotional states in a straightforward way. Do not limit yourself to a dark palette, however. Artists such as Cindy Sherman use bright, sickly, garish colors to create disturbing effects. If you are a literary artist, use colors in your prose to exemplify characters' dark feelings and actions.
Employ metaphors to get across dark themes in your work. A metaphor is a tangible thing that represents something less tangible. For instance, a tattered baby crib might stand for a mother's disappointed hopes and dreams or a child's horrific upbringing. Think of strange, surprising metaphors rather than relying on familiar ones. Everyone knows that a skull and crossbones represents death, so it does not make an interesting metaphor.
Use irony to portray darkness in a fascinating way. Irony involves a situation, image or statement where the literal meaning differs markedly from the intended meaning. For instance, a perfect prom queen, with her shellacked hair and lipsticked smile, might portray the emptiness and soul-crushing despair of high school conformity. By using images and words to suggest the opposite of what they seem to mean on the surface, you will create a rich and multilayered work of art.
Be direct. Simply tell, paint, photograph or otherwise record your dark tale in the most straightforward way possible. Write down exactly what you feel or draw a picture showing what the world looks like through your eyes. Use this technique to brainstorm new ideas or to break through creative blocks when you start a new artwork. When you get further along in your work, you can incorporate rhetorical devices, such as irony and metaphor.