Heat tint (or temper color) provides insight into sheath temperature while in operation. The surface oxide layer thickens and reflects a different spectrum of light. In heaters, it is indicative of poor fit, improper tolerancing and/or incorrect specification.  The image below shows a heater that experienced temperature spikes due to a damaged bore.  The improper fit negatively affected its ability to transfer heat. Just by looking at the color, we can tell that the middle of the heater reached or exceeded 600 °C / 1112 °F.