Natural ground color is useful for reference maps as well as maps where a realistic representation of the Earth's surface matters. Natural color schemes are less likely to be misinterpreted, as opposed to hypsometric color schemes, and are generally preferred by map readers. The creation of natural-color maps was once limited to manual cartographic techniques, but they can now be created digitally with the aid of raster graphics editing software. However, the creation of natural-color maps still requires many steps, a significant time investment, and fairly detailed digital land cover information, which makes this technique impossible to apply to global web maps at medium and large scales. A particular challenge for natural-color map creation is adjusting colors with location to create smoothly blending transitions. Adjustments with location are required to show land cover transitions between climate zones with a natural appearance. This study aims to take the first step in automating the process in order to facilitate the creation of medium- and large-scale natural-color maps for the web. A coloring method based on two grid inputs is presented. Here, we introduce an algorithmic method and prototype software for creating large-scale web maps with this technique. The prototype software allows the map author to interactively assign colors and design the appearance of the map in an automated way. This software can generate web map tiles at a global level for medium and large scales. Example natural-color web maps created with this automated coloring technique are provided.