I2C Sensors have a default address. The address allow for multiple I2C devices to be one the same "bus" but each device need to have a unique address. If you have two color sensors (its not limited to just color senors, it is if you a more than 1 I2C device with the same default address), you have to change the address of the one of the sensors.
There is a sample OpMode, LinearI2cAddressChange, that shows how to change the I2C address of the Modern Robotics IR sensor. The same technique can be used for the color sensor. Although comments on the forum stay that this solution is not quite complete.