The value 6,500K marked on fluorescent lamps represents a parameter called correlated colour temperature.
It means that the spectral, or light, colour distribution from that lamp is similar to that of a black body at that temperature.
Any black body when heated emits different colours at different temperatures.
At 2,000K it emits red light, at 4,000K it is yellowish white; at 2,700K, it provides warm light; and at 6,500K, it gives the impression of cool daylight.
Based on colour appearance, fluorescent tubes are classified into three types: daylight white (above 5,000K), neutral white (4,000K), and warm white (below 3,300K).
The colour of light depends on the fluorescent coating inside the tube. Three types of coatings are generally given: tri-phosphor, standard phosphor, and multi-phosphor.
Tri-phosphor coated lamps, like incandescent lamps, emit yellow light similar to sunlight.
Standard tube lights render a colour similar to daylight.
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