Wallhaven is really great for finding images, however I feel that the search filtering is limited in only allowing color filter, and not luminosity. This makes it difficult to filter down to a specific type of image.
For example, in a random search where you select black as a color, you get images like this and like this.
Subject matter aside, this makes the it hard to find wallpapers that for example contain the color red and are very bright, or contain violet and are very dark.