Try adding a delay(1000) at the end of matrixWriteBit(). But before you do that, you must add those series resistors, otherwise you could blow your leds. The delay will slow down the multiplexing, so that you can see the true brightness of the leds. They may be dim because of the multiplexing, which with your sketch above is using a 1-in-16 duty cycle. With code changes, this can be reduced to a 1-in-4 duty cycle.