Although most lamps emit 'white' light, this can vary from a cosy 'warm white' to a cold or rather 'cool white', according to the 'colour temperature' of the lamp. Colour temperature is measured in degrees kelvin and denoted by a numerical figure followed by the letter 'k'. Somewhat confusingly, and in complete opposition to the celsius scale, lower values equate to warmer colours and vice versa, as can be seen in the chart below...
The values in this scale apply to some of the more common colour temperatures found in modern lighting applications. Some bulbs are tied to a specific colour temperature because of how they work, such as incandescent bulbs that use heat to generate light and therefore naturally produce a warmer glow than other bulbs. Other technologies though - such as LED - can accommodate multiple colour temperatures, from warm white right through to daylight and beyond.