There are two models: "White ambiance" and "white and color"
White ambiance supports 2200K-6500K natively - f.lux will use dimming below 2200K.
White and color support 2000K-6500K natively - f.lux will emulate warmer colors on these models using the color capabilities, so usefully 1200K-6500K works.
Both models achieve their peak melanopic value somewhere below 6500K so we do not go to efforts to make them more blue than this - i.e. we could do it with the "color" models but they would get a lot dimmer, so we did not do it yet.