This paper proposes a pump wavelength assignment scheme for an optical packet switch with parametric wavelength converters (PWCs) to reduce the number of different colored light sources while a predefined packet loss rate is satisfied. PWCs are separated into two sets. First set, each PWC has a different pump wavelength from each other. Second set, all PWCs have the same pump wavelength placed at the center of the transmission wavelengths. Increasing the number of PWCs in the second set reduces the number of different colored light sources. However, the packet loss rate increases. Therefore, the number of PWCs in the first set is minimized.