We have theoretically modeled the “giant” colloidal core–shell quantum dot with strain adapting alloyed interfacial layer between core and shell materials for optoelectronic applications. The intersubband optical properties, such as absorption coefficients (ACs), refractive index changes (RICs), oscillator strength, and transition lifetime of giant CdSe  /  CdSe_xS_{1  −  x}  /  CdS core–interfacial–shell quantum dot (g-CISQD) have been investigated for different alloying “x” of the interfacial layer with varying core radius. The results have been plotted considering the significant impact of strain between the heterojunctions and compared for different structural parameters. The results show that the oscillator strength and transition lifetime strongly depend on alloying x and core radius. The energy eigenvalue in g-CISQD decreases with increasing x and saturates beyond the value x  =  0.5. The resultant redshift of the resonant peak in ACs and RICs is concurrence with the obtained energy eigenvalues. Further, the ACs and RICs have been investigated for different intensities of incident light   (  I  )   and core radius variation. The results provide the tuning possibility of the optical properties using appropriate alloying x and core radius for application in optoelectronic devices.