Why wouldn't you see blue or purple when you heat metal and only either red or yellow?
Because hot bodies emit a continuous range of light not a single wavelength

All warm objects emit a range of wavelengths that depend on how warm they are. The source of the light is a variety of mechanisms not all of which involve electronic transitions (some will involve transitions between molecular or solid state vibrations or rotations.) The important point for the perceived colour of the light they emit, though, is that the emitted radiation is continuous.

The chart below show the mix of light wavelengths from bodies at different temperatures:



The peak of the emission curve changes with the temperature. So the mix of light you see does change colour somewhat with the heat of the object but will never be a pure colour like blue or purple. Tungsten light bulbs, for example, reach temperatures of 2.5-3K °C and have peak emissions in the infra-red; their light has more red than blue in the visible so they are perceived as somewhat orange when compared to sunlight (which follows the curve for a body of 5-6k °C, the sun and has peak emissions in the middle of the visible spectrum).

The differences between different sources of black body radiation are harder to perceive than you might think because the eye and brain are good at adjusting what you see to a common perceived colour mix and it is only when comparing different light sources that the differences in the mix of light become easy to see (something also obvious when comparing other types of light bulb to tungsten bulbs).

So a hot object at ~7k °C will emit more blue light than red light but it will still be emitting a lot of red light so you will tend to see blue-tinged white rather than pure blue. Astronomers can, when their eyes are well adjusted to darkness, tell the subtle colour differences of different stars (which range in temperature from maybe 2k °C to maybe 20k °C). But the effect is small.

Of course, if you heat something enough you will turn it into a vapour or plasma and there will be enough energy to excite the outer electrons. The light resulting from transitions of outer electrons is often very narrow in wavelength and can be used to identify the atoms in the vapour. Low pressure Sodium street lights (the bright orange ones) are a result of this effect (the excited outer electrons sodium atoms emit yellow-orange light very characteristic of sodium). Many other elements have characteristic lines that can be just about any specific colour under similar conditions). 

But normally when you heat things up you just see the continuous black-body emissions and that can't be a single colour.