It doesn't really change color. As tars build up inside the piece the colors of the glass show differently. When you clean it you go back to square one. http://knowledgebase.grasscity.com/topics/whats-color-changing-glass/
the buildup of resin provides a darker background, making the glass appear to be different colors. so it will change after use, and after many uses you should notice a rather dramatic change depending on how much fuming was done
hmm, i thought it had something to do with the heat. Ive had pieces that were color changing, then when i scraped it, it didnt look the same, but it also didnt look like it did when i first got it. I thought it was a mix of heat and the resin build up?