In this work we present a new operational principle of an adjustable, high resolution color sensor. The concept is based on a single photodiode, which was specifically designed for a variable spectral response, corresponding to the extension of its space charge region. A wide range of spectral sensitivity functions (i.e. the receptor function of the human eye) can be emulated from a series of measured photo currents under different voltage bias conditions. The variable photodiode was realized in hardware, and its functionality for true color sensing was confirmed by electro-optical measurements and algorithmic transformations. The new tunable photodiode enables a very flexible sensor operation. It allows for modifications of the functionality of the sensor, which is defined by its spectral response function, even during operation simply by changing the measurement protocol and algorithmic processing e.g, software adaption. No change in the hardware configuration of the sensor is necessary.