My understanding is that the rainbows from DLP projectors stem from the speed of the color wheel and also how many color segments that wheel may have. If you look at some specs for the projectors, you'll see that the higher end the projector, the faster the speed of the color wheel and also how many color wheels may be involved.

For more helpful info, I would try looking up DLP projectors at the following two sites:

www.avsforum.com
www.ecoustics.com

Good luck!