The Night color dropdown is about better sleep. The lower the value of the Night color the less blue light is emitted by the screen.
Blue light is bad for your sleep, because it stops the secretion of melatonin, our sleep hormone. I set my night color to 2700K at night, but the rule is the lower the better.
Note that under Windows there is limitation of this color changes by default and Iris will prompth you to Expand your color range on the initial calibration.
With the new versions of Iris this expanding is automatic and you just need to restart your computer once after the first Iris installation.
If some of the values on this dropdown are unavailable chanses are you need to Expand your system color range.
By default also Iris will apply this color changes only at night and will use your location to find the sunset time. If you want to manually control the Color temperature set Manual color settings to ON.
After every change remember to click Save and close otherwise all changes will be reverted to the last saved state.
So for good night sleep set this settings to low value. The lower the better.