The secret to color changing glass is actually the fuming. Fuming is the process of vaporizing a precious metal (silver, gold, platinum) onto clear glass. This atomized metal is what causes the glass to appear to change color.

How do Colour changing bongs work?

When the pipe is used resin builds up and interacts with the ions from the fumed metal and starts to change colors. The more you use the pipe the more it will change colors. Here is a fun fact, the glass itself actually never changes colors so when you clean the glass it will go back to how it was the day you got it.

Why do glass bongs change colors?

As you smoke your pipe and it gets dirty from the resin, the black resin does not allow the light to pass through the glass. This allows the light to almost reflect back through the gold and silver fuming. As this happens over time from smoking, the pipe will appear to change color.