We present a user interface design for labeling elements in document images at a pixel level. Labels are represented by overlay color, which might map to such terms as "handwriting", "machine print", "graphics", etc. The primary purpose is to streamline processes for manual production of groundtruth data, which is necessary for training algorithms and evaluating performance. Unlike general painttype programs, the UI design is targeted specifically toward selection of collections of foreground pixels that are likely to be meaningful elements in a document image analysis context. Our implementation, called PixLabeler, is available for download and allows customized plug-ins for bootstrapping according to the labeling task.