We report the growth and characterization of patterned ZnCdSe structures on GaAs substrates as our initial attempt to use shadow mask selective area molecular-beam epitaxy (MBE) to integrate II-VI (Zn,Cd,Mg)Se-based red-green-blue (R-G-B) light-emitting diodes (LEDs). Patterned ZnCdSe thick layers and ZnCdSe/ZnSe quantum wells (QWs) were grown on GaAs substrates using a silicon shadow mask mounted on a mask fixture that allows the mask to be placed and removed within the MBE growth system. Excellent pattern definition and good optical properties were obtained. Integration of patterned ZnCdSe/ZnSe QWs having different thickness and Cd composition, therefore different emission wavelengths, on a single GaAs substrate was also achieved. These results will be applied to the (Zn,Cd,Mg)Se material system to integrate R-G-B LEDs on a single InP substrate.