In order to marble the fondant, you need 2 colors to mix. I started with this pink and added the blue to it for my marble on this cake.
Twist them together and stretch, repeating a few times, but not too many!
If you don't have 2 colors, you can add colors this way. Add a dab of your usual cake colors in several places, folding as you go.
Knead and twist as I showed with the 2 color option.
I forgot to take a picture of this blue cake, but I've included a picture of another marbled cake I did.