You’ll notice a few tools in Lightroom that have a Color Picker built right inside of them. One place is in the Split Toning panel in the Develop module. Also, the Graduated Filter, Adjustment Brush and Radial filter all have them so you can paint or target a specific part of the photo to have a color tint added to it. Here’s an example of the color picker (circled below).

As you’d expect, when you click to see the color picker you can just click to sample any color from it that you want. But there’s actually a secret little tip that let’s you sample from a color in the photo if you wanted to match the same tones as your photo has. See, if you simply just click on your photo to try to sample the color, Lightroom will close the color picker box (rather than setting the color you want). So here’s the trick. Click inside the color picker first. Don’t let go of your mouse button yet though. Then just drag (while still clicked) outside of the picker and over your photo to force Lightroom to sample the color that your cursor is over.
Pretty cool huh? It’s definitely not one of those little things you’d ever stumble on, but if you ever need to match the color in your photo (without just guessing), now you’ve got a tool to do it. See ya!