What it shows:
The thermal radiation emitted from a black body is temperature dependant. The tungsten filament of a bulb approximates a black body, and by increasing the current through the bulb, its temperature rises and so its color spectrum shifts to shorter wavelengths.
How it works:
We use a 1.5 kW GE bulb connected to 110V through a variac. Starting at zero you can gradually increase the current through the bulb. You get a clear change from orange-red to orange to yellow before the bulb becomes intense white.
Limited by the characteristics of the bulb. It gets unbearably bright too soon to get a decent range of colors. Any size clear bulb will do for this.