These values are fine-tune adjustments for the selected white balance preset, not direct Kelvin numbers.

On the D40, moving the setting toward "+" makes the image warmer, while moving it toward "-" makes it cooler. They’re relative steps designed to shift white balance in perceptually even amounts rather than fixed Kelvin increments.

Technically, this kind of adjustment is based on mired shift rather than simple Kelvin steps, because equal Kelvin changes do not look equally different across the scale.

In practical use, +1 or +2 can make skin tones or autumn colors look warmer and more pleasing, while -1 or -2 can give snow or winter scenes a cooler look. The 0 setting is the camera’s neutral/standard value for that preset.

This is most useful if you shoot JPEG, since the white balance is baked into the file. If you shoot RAW/NEF, you can fine-tune white balance later in post-processing with little or no penalty.