depends on pan sharpening method you are using, there were some methods that relatively will not alter spectral properties of the multispectral bands when they are pan sharpened. Some of them are Ehlers FFT pan sharpen (implemented in ERDAS IMAGINE), Gram Schmidt Spectral Sharpening (implemented in ENVI), Yun Zhang UNB's Sharpening (implemented in PCI Geomatica), SFIM sharpening (implemented in ER Mapper), and Wavelet transform (Erdas Imagine, PCI Geomatica).