The modulation instability of two broad beams with different frequencies is investigated in a photovoltaic photorefractive crystal with a uniform background illumination. It is shown that modulation instability of broad polychromatic beams can be adjusted and controlled by the Glass coefficient ratios (r1, r2) of the signal and background beams. It is also found that the strength of modulation instability depends on the ratio of beam intensities f and g. For the case of r1>1,r2>1, the maximum modulation instability gain increases first and then decreases with the increase of g when f<0.5, while it decreases monotonically when f>0.5. For large g, the modulation instability is suppressed and tends to be 0. For the case r1>1,r2<1, the signal beam whose r2 is less than 1 can suppress the modulation instability of the other signal beam whose r1>1 when f<4. If f>4, the modulation gain is always very small. For the case r1<1,r2<1, the polychromatic broad beams do not undergo modulation instability.