The spectral red-shifting observed in homogeneous polar solvents can be well explained by the larger dipole moment of the S 1 state compared to the S 0 state. A larger dipole moment of the S 1 state leads to a stronger stabilization effect by the polar environment, which decreases the energy difference between the S 1 and S 0 states, thus causing a redshift of the emission wavelength. In the implicit solvent model, the environment is homogenous in any direction, and the interaction exerted on the chromophore correlates with its total dipole moment. In contrast, the effect of OEEFs is directional and correlates with the component of the difference dipole moment along the specific direction of the applied field. In the current case of LFOH or HFOH, the charge redistribution upon S 0 to S 1 promotion mainly occurs along the long axis of the isoalloxazine ring (X-axis). Accordingly, the spectrum is most sensitive to F X and much less sensitive to F Y and F Z . Qualitatively, applying a field along the direction of the fieldfree difference dipole moment decreases the emission energy E F and thus leads to the redshift of the emission maximum λ F , while an opposite field increases E F and blueshifts λ F . Within the field strength applied in this study (0.0075 a.u.), the effect of OEEFs obeys the first-order Stark effect. The emission energy E F responses linearly to the electric field, and its variation is proportional to the component of the field -free difference dipole moment along the field direction. Therefore, a field along the direction of the difference dipole moment is most efficient for spectral modulation. The linear and continuous dependence indicates the feasibility of intentional and accurate color tuning through control of the local electric field. As for intensity, the substantial enhancement in polar solvents comes from the increase of the transition dipole moment. Notably, in the case of F X , E F experiences a larger variation compared to the case of polar solvents, which in principle would lead to a considerable change in intensity. However, the variation of the transition dipole moment offsets the change induced by E F , resulting in only a slight variation of intensity under F X .