Gray scale art is heavily toned with grays. They are effectively used to establish a color scheme in a black and white book. They also create three-dimensional shapes, separate planes in a panel, and add textures to items that would otherwise appear flat and clean. Taking those in order: The grays establish a color scheme just by helping to differentiate between tones. In some cases, darker tones indicate a night scene or a shadowy room. The three dimensional shapes occur in places where the tones wrap around items in a scene. Since there are elements of noir crime drama in here, light pouring in through window blinds is a necessity. Those stripes of light, in one scene, are wrapping around the sleeping body of the protagonist, giving him extra shape. It's obvious that his arms aren't flat even before the toning, but the inclusion of the tones emphasizes the point and helps to create a more naturally "real" feel. The traditional use of gray tones in indie comics is to keep the pages from looking too stark or simple, as well as popping things out of the panel. A brighter-lit object will pop off the page a bit more than a darker object set in a dark scene. It's a cheap and easy way to indicate this, in addition to varying line weights. It's one of the hardest things a colorist has to do in comics -- create a color scheme that varies from the literal to the metaphorical. No, that character isn't blue-skinned, but if the blue helps establish the mood and place the figure in the right spot in the "space" he's occupying, then it's the right way to go. Finally, some gray tones can indicate texture. I'm looking at characters who wear jeans in this book and see the way soft gray lines are used to indicate folds in the cloth. I see the shine off a pair of gloves a character is wearing, and I know he's wearing leather gloves and not latex gloves on a crime scene.
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