I had to look this up...

Temper Color

the iridescent color that appears on the smooth surface of hot steel as a result of the formation of a very thin oxide film. The thickness of the film depends on the reheating temperature; films of different thickness reflect light rays in different ways, which accounts for the different temper colors. Typical temper colors for carbon steel are straw (220°C/428°F), brown (240°C/464°F), purple (260°C/500°F), dark blue (300°C/572°F), and light gray (340°/644°F). The same temper colors appear on alloy (especially highalloy) steels at higher temperatures. The colors are also affected by the holding time of the steel at a given temperature, the illumination, and other factors. Before the advent of pyrometers and suitable testing and measuring instruments, the temperature of reheating was judged according to the temper color.

If it's yellow... let it mellow.
If it's brown... flush it down.

Mine's got the full rainbow.... it happened in the first few days I owned it,.. whilst I wrassled with the leaky pet problem...
I left it on,...
...(makes me look like a badass)... ...

I've given up trying to be a bad ass.  Too many people would just point and laugh.   Is there a substance you can buy to remove  the coloring, seems like I saw a product that claimed it could.

Thanks Versagen1. I may need to get my eyes checked. At first I thought you typed something rude.  

Why bust a gut over a blu pipe? It looks good, once ya get used to it,