I want to pass along a tip on how to sample a color from the original photo when applying a split tone affect to a photo that has been converted to grayscale in Lightroom.
First, go to View > Before and After to choose a view that puts both the original color version of the photo along with the current grayscale version on screen together.
Next, click the color swatch for either highlights or shadows to open the color picker. Click and drag from the color picker out on to the color photo. Release the mouse to select the color you are sampling. You will see a live update as you move the cursor over the screen.
The great thing about this functionality is that it works with every place in Lightroom where you find a color swatch. You can even sample colors from any other application or area of your screen. Give it a try!