These are black body radiation curves,
Most materials, including metals, obey the black body curves whose areas contain the total energy radiated and are a function of temperature.
For 1250 K most of the radiation is in longer wavelengths than the visible. At 1750 K the red visible wavelengths are present , hence "red hot metal". As temperatures go higher, more short wavelengths enter.
The smaller wavelengths belong to higher temperature sources, as sparks and stars.
As the whole visible spectrum is accessible to the eye in this case, they seem "white" by the color perception mechanism of our eyes.
The scale in your second figure shows the continuous accessing of smaller wavelengths as the temperature goes up. From red to yellow visible wavelengths are increasingly accessed. At the temperatures listed on the right, the amount of energy in the visible spectrum is very small, but enough so that it is recorded in the plot. It says in your link:
These colors were obtained from a 0.40 wt. % carbon, alloy steel, as seen through a furnace peep hole during average daylight conditions.