If you want to use digital ink in your document, there are features available on the Draw tab in Microsoft Word that will help you to do that. The Pen feature will allow the user to draw on the document with ink,  and the Highlighter feature will allow users to highlight a text with ink. The Pen and Highlighter contain features to choose the thickness of the ink you want and choose a color; If you want to see more colors, you need to click the More Color feature. You can choose a standard or a custom color.