Cheap RGB-D sensors are ubiquitous in robotics. They typically contain a consumer-grade color camera that suffers from significant optical nonlinearities, often referred to as vignetting effects. For example, in Asus Xtion Live Pro cameras the pixels in the corners are two times darker than those in the center of the image. This deteriorates the visual appearance of 3D maps built with such cameras. We propose a simple calibration method that only requires a sheet of white paper as a calibration object and allows to reliably recover the vignetting response of a camera. We demonstrate calibration results for multiple popular RGB-D sensors and show that removal of vignetting effects using a nonparametric response model results in improved color coherence of the reconstructed maps. Furthermore, we show how to effectively compensate color variations caused by automatic white balance and exposure time control of the camera.