In molecular biology, fluorescent proteins have become a unique marking tool for gene expression, environmental pollutants, and monitoring the dynamics of AIDS virus and single-molecule motors [1-3]. Particularly, the mutagenesis method is employed to produce mutants of fluorescent chromophore for color shifts [4, 5]. However, due to the uncertainty and complexity of the mutation effect on the wavelength of emitted light, real color modulation for target marking remains unavailable. In this work, we report a top-down method for the accurate and continuous color tuning of firefly chromophore (oxyluciferin) by controlling the surrounding polarization electrostatic fields. Systematic investigations of the absorption spectra of oxyluciferin molecules are carried out in the framework of time-dependent density functional theory. Results show that the polarization electrostatic field applied on the long molecular axis significantly changes the optical properties. However, if the field is applied on the out-of-plane axis, its effect is almost negligible. Under long axis electric fields, the wavelength of the two main peaks shifts continuously, covering a wavelength range of about 100 nm. Such a wide range of wavelength shift provides us a realizable modulation technique for very accurate color tuning of fluorescent proteins. The need of any special marking application can be met by careful design of the local polarization electrostatic fields. On the other hand, the peak intensity is also associated with the electrostatic fields, which shows that the efficiency of light emission can be well enhanced as well. [1] M. Chalfie et al., Science263, 802 (1994). [2] K. F. Sullivan et al., Green Fluorescent Proteins (1999). [3] D. W. Pierce et al., Nature388, 338 (1997). [4] B. R. Branchini et al., Biochemistry43, 7255 (2004). [5] A. Moradi et al., Biochemistry48, 575 (2009).