Artificial light is rated according to its color temperature, listed as a Kelvin number (Celsius degrees).
When you heat up an incandescent filament, it radiates light. At lower temperatures, the filament gives off a more orange or yellow light. As it gets hotter, the color it radiates becomes bluer.
When you buy a bulb, it is rated on this scale. In this photograph, a series of bulbs are lined up in a gradation from 1000 K to 10,000 K. Although the scale was created based on incandescent light, it is used for LED and fluorescent light as well.
A candle flame is about 1900 K. Bulbs that produce light at 2-3,000 K are often called "warm white" in the industry. White, neutral sunlight is rated at 5,000 or 5,600 K. Studio north light is closer to 6500 K and above that, the pure blue sky can go all the way up to 10,000 K.
This scale can be confusing or counterintuitive for artists, because the bluish paint colors that we call "cool" are associated with the light that is emitted at higher temperatures, while "warm" colored light comes from cooler sources.