As Bro DZ said, the only thing that causes the color change is the build-up of resin (that black gunk) on the inside of the pipe. The more resin there is, the less light can get through the pipe, which is why it appears to change colors. Fact is, you'd have a tough time keeping it from changing colors...you'd have to clean it all the time so the resin never built up. If you use the rubbing alcohol, make sure you leave it out so the alcohol has time to evaporate, and then it will be good as new and you can start watching the resin accumulate and the colors change again.