keep in mind that the tester that i posted will not blink to any programmed sequence.
but will light up one color for a duration with the passing of the 1's, and the other led lighting up for the duration of the passing of the 0's.
so they will both light up, and might be different intensities, and might even just stay that way, or change because of the bits in the data stream. if you build a tester out of a bi directional led, it will appear as a blend of the two colors.
the stream is never purely 1's or 0's, so one light wont be one, and the other off. but if no signal was present, then it wouldn't light at all. so it's more used as a DMX is present detector, than anything else, and, i think, is similar to the green light in the dongle.