Colour can also be indicated in other ways.

Colour Temperature is the temperature that a theoretical perfect radiating black body would have to be heated to in order to make it radiate light at a specific colour.  No such "perfect" body exists, but if you think of the way that iron glows as you heat it in a forge, you'll realize that real objects can behave in a similar way.

Since light is an oscillating electromagnetic field, it also has an associated wavelength and frequency.  The following table indicates the approximate wavelength, frequency and colour temperature for various colours...

Table 3 - Colour temperatures