Yes. Split Toning works by adding a hue to the highlights and/or shadows. Gray, white, and black are not hues—they’re neutral tones with little or no saturation—so they aren’t something a hue selector can target directly.

That’s why Lightroom’s Split Toning control only offers colors around the hue wheel. If you want parts of an image to look more gray, you’d typically reduce saturation instead, such as with HSL controls.

Brown is a bit different: it isn’t usually treated as its own separate hue in tools like this. It’s generally perceived as a darker, less saturated orange/red tone. So to create a brownish toning effect, you’d usually choose an orange or red hue and lower the saturation or brightness until it looks brown.

In short: Split Toning selects hue, not neutral tones, and brown is usually achieved by adjusting an existing warm hue rather than picking “brown” directly.