LEDs are normally rated in nano meters of the band width of light they emit .On the visible spectrum red is the lowest at 620 < λ < 645 nano meters and blue the highest at 460 < λ < 490 nano meters.Although we can't see UV light it does enhance brilliance to the broad spectrum .White leds are a broad spectrum of either Blue/UV diode with yellow phosphor for Cool / Pure White or Blue diode with orange phosphor for Warm White or RGB color mixing but RGB LEDs leave gaps in the spectrum .Because of these limitations LEDS don't have a kelvin rating .
Other sources of light like halogens can be adjusted very finely by changing the amounts of various gases to achieve varying warmth's hence a broad range in kelvins.