Whilst the Drag and Drop editor will allow you set link styles for the whole message, to do this, select the icon from the menu on the right hand side which looks like a capitalised A.
After you click on this, the menu will open and you can then select 'text links' within the options. This will expand that field and allow you to set a global setting for the message for link format.
You may decide to change the colour of an individual text link. You highlight it and change the colour, like any other text.