How do I change the highlighter color in Adobe Acrobat Reader DC permanently?

You can change the highlight color using the Highlight Text tool, which is available from the (rather hard to find) Comment Toolbar like this:

1. Go to View > Tools > Comment > Open to enable the Comment Toolbar
2. Click on the Highlight Text tool button
3. The Color Picker button now turns from greyed-out (disabled) to blue (enabled)
4. Two options here:

• Option 1. Click on the Color Picker button and select the color you want your Highlight Text tool to be (limited colors).
• Option 2. Ctrl+E (Windows) or command ⌘+E (Mac) to open the Tool Properties menu (broader selection of colors, as others have pointed out).

4. Go forth and Highlight text

Screenshot for context:

Here's the easiest way: Ctrl + E

(If the "Highlight text" tool is not selected, select it first by clicking on it.)

Now you can choose your favorite color from color panel at left corner box