This paper describes a database collecting natural color images, called the Natural Image Database, and shows how it is used for illuminant estimation problems. First, we present significant features of the database. This database is not only a collection of color images of natural scenes, but also includes scene illuminant spectra and camera calibration data. Because these data sets are available via the Internet at http://www.osakac.ac.jp/labs/shoji, the database can be used as the calibrated common test data for evaluating the performance of illuminant estimation algorithms. Second, we focus on two illuminant estimation methods, the subspace method for estimating scene illuminant spectra and the sensor correlation method for classifying illuminant color temperature. The algorithms are briefly described. The former is based on the linear model representation of possible illuminant spectra, and the latter is based on correlation computation between an observed image and the reference illuminant gamuts. The performance of these algorithms is evaluated using the database.