This used to be very common with the early TI chips; I know when I bought my projector in '05, I demo’d a bunch and I saw the rainbows on the DLP Projectors… thus why I went with LCD.
Now, I’ve looked at some new projectors (debating a serious upgrade), and I no longer see the rainbow effects like I did with the older models.
Something I read about TI changing the way the mirrors were on the chip and the movement of them. Not totally sure though