Steel begins to glow faintly red at about 600 degrees celsius. A very slight red color may be visible below this temperature if the light is dim enough.
Steel is still relatively soft just after the red color fades but at this point it rapidly get harder as it cools.
In bright light, e.g: sunlight, the red color may not be visible until the steel is well into the "cherry red" temperature: more than 750 degrees celsius.
Bright orange indicates a temperature of 880-980 degrees, while yellow is more than 1000 degrees. Note that the melting point of plain carbon steel is around 1300 degrees; at this temperature the steel is is a brilliant whitish yellow, too bright to look at.
In my experience, in bright sunlight, even molten steel appears yellow to orange yellow.
Warning- Above the cherry red temp, I strongly recommend wearing welding goggles, perhaps a #4 or #5 lens. Glowing hot steel gives off intense infrared rays which cannot be seen, but it can still damage your eyes.
I work as a welder.