A color tuning index (I-CT) parameter for evaluating the color change capability of color-tunable organic light-emitting diodes (CT-OLEDs) was proposed and formulated. And a series of CT-OLEDs, consisting of five different carrier/exciton adjusting interlayers (C/EALs) inserted between two complementary emitting layers, were fabricated and applied to disclose the relationship between I-CT and C/EALs. The result showed that the trend of electroluminescence spectra behavior in CT-OLEDs has good accordance with I-CT values, indicating that the I-CT parameter is feasible for the evaluation of color variation. Meanwhile, by changing energy level and C/EAL thickness, the optimized device with the widest color tuning range was based on N,N'-dicarbazolyl-3,5-benzene C/EAL, exhibiting the highest I-CT value of 41.2%. Based on carrier quadratic hopping theory and exciton transfer model, two fitting I-CT formulas derived from the highest occupied molecular orbital (HOMO) energy level and triplet energy level were simulated. Finally, a color tuning prediction (CTP) model was developed to deduce the I-CT via C/EAL HOMO and triplet energy levels, and verified by the fabricated OLEDs with five different C/EALs. We believe that the CTP model assisted with I-CT parameter will be helpful for fabricating high performance CT-OLEDs with a broad range of color tuning. (C) 2015 AIP Publishing LLC.