How do you create a brown color in Lightroom

Or like this using split-tone. Eyeballed, takes a little practice/fiddling. As others have noted, you can't think of painting over. The color you want might not become apparent to you until you see it in a darker or lighter part of the image.

JaovdL's suggestion is the way to go for b&w colorization. I'd add to not be afraid to use different colors and amounts in highlights and shadows. For example a brown (40) in the highlights can be offset by a blue (220) in the shadows. With judicial choice of saturation of both combined with balance, this can add a richness that can be subtle or bold.

Best way to colorize a B&W is with the split toning shadows sliders. Brown is around a hue of 50 and you don't want to overdo the saturation. Playing with the tone curve helps.

If you are after a close match to a known color, you can use the color picker dropper tool to select a color from anywhere on the 'desktop'.

Normally you would click on the color "x" control as advised by joefry99 and choose from within the color dialog that opens.

But if you click and hold the left-mouse button down, the dropper can be dragged to anywhere on the screen, and when you let the mouse button go, it selects the color under the cursor.

One important note about color in Lightroom- it will only apply a color TINT to the area you adjust, it does not 'paint' 100% color pixels where you apply it. And the tint will only show on areas in a photo that have some tone. As in my screen-clip: the white areas of the photo do not tint (with the Gradient filter)

The ability of Lightroom to ccreate colors is limited. The original colors of the photo can be adjusted within some range, but you cannot go beyond that range. You can't turn green to red, and you can't turn white or grey or blue or red or green to brown. This seems like a task better suited to Photoshop.

You might find this helpful: click on the color "x" control at the bottom of your gradient filter. This allows you to dial in an overall hue and saturation to the whole filter. Combine with the normal color adjustments on the filter to taste.