Whilst the Drag and Drop editor will allow you set link styles for the whole message, you may decide to change the colour of an individual text link. You highlight it and change the colour, like any other text.
If you notice that when you have highlighted and changed the colour of the linked text, it has not changed the colour of the underline, there is a little change you can make to update it.
Click on your linked text then open the edit link menu again and switch to the advanced tab. In the ‘Style’ text box at the bottom, simply type or paste “text-decoration:inherit” then hit OK.
This will make your link use the styles you have added to the text and it will hide the underline. If you want it back, highlight the text and click the to add the underline again.