There's a few different ways you can change the color of links in your email campaign (Fig 1).

Fig 1

Going from top to bottom, if you adjust the Color Slider and then click on the large swatch box above, you can select what shade of color you want. 

Alternatively, you can manually type in the hex or RGBA code (clicking "hex" toggles which kind of code it is) to pick a color. 

If you click the link after creating one, it'll bring up the Link Menu (Fig 2). 

Fig 2

From here, you can change column options, link style, and link options. 

Alternatively, you can click on the text widget, which opens the Text Options modal. This is another place you can change the link color (Fig 3).

Fig 3