Temper Colors
Temper colors are the thin, colored oxide layers that form on bare steel when heated. Toolmakers read the tempering temperature from them — from pale yellow at 200 °C through violet to gray at about 400 °C.
When you heat bare steel, a thin oxide layer forms on the surface. It's so thin that light refracts off it like a soap bubble — and depending on the layer thickness, it shimmers in a different color. These are temper colors, and they're a great tool: The layer thickness directly depends on temperature. If you slowly heat a bare tool, you'll see the color travel across the surface and you can simply read the tempering temperature.
Temper Colors
How to read the scale: Pale yellow at around 200 °C means high hardness (knives, punches, cutting edges). Straw yellow to golden yellow around 220–240 °C is the classic for cutting tools. Violet and cornflower blue at 300–320 °C already mean noticeably less hardness, but more toughness — the range for springs and tough components. From gray-blue and gray (above 350 °C) the steel is soft-tempered.
Two pitfalls: First, this only works on a bare, clean surface — on or rust you won't see any temper colors. Second, don't get confused: The heat tint on a stainless steel weld is something different — there, the discoloration shows how much the weld has oxidized and how much was lost.