A voltage-tunable four-color quantum-well infrared photodetector (QWIP) has been demonstrated. The advantages of this detector include widely separated peaks, narrow linewidths, and negligible spectral cross-talk. The four colors demonstrated are widely distributed from mid-infrared to long-infrared, and are centered at 4.5, 5.3, 8.3, and 10.4 μm. Each spectral peak is well defined and the corresponding linewidth is 0.58, 0.71, 0.83, and 1.01 μm, respectively (corresponding to Δλ/λ<0.14). The detector structure consists of two stacks of superlattice materials that are separated by a middle contact layer. Each material is designed to detect two specific wavelengths, tunable by the bias polarity. Four-color detection is achieved by applying different combinations of top and bottom bias relative to the common middle contact and reading out the optical signals sequentially from these two contacts. Using an appropriate bias sequence, the detector can be made sensitive to only one color at a time and the signals from the other colors are negligible. With this detection scheme, one can select different combinations of detection wavelengths and linewidths over a wide spectral range. This design thus greatly increases the flexibility of QWIPs in multicolor imaging.