To expand on this, the rainbow of colors which start at a straw then turn yellow, red, brown and then that vivid blue, are caused by refraction. The oxide layer on the surface is transparent or translucent, and the thickness of the layer determines what wavelength of light it scatters. The hotter it gets, the thicker the oxide layer forms, so you can fairly reliably tell the temperature the metal has been heated to by eye, and you might use different amounts of heating to achieve hard-but-brittle or soft-but-tough.
I've even seen it done by Chris of Youtube channel Clickspring for decorative purposes. It's how he made the steel hardware of his brass clock blue.
Exactly how you temper something the size of a sword using a forge is a bit outside my understanding; I've done it with relatively small bits of drill rod to make lathe tools with a gas torch, but that's about it.