I just made my first fondant cake (see link) and had a very hard time getting the color seamless.

http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1199362

My green looks like a tie-dyed mess. Any tricks of the trade to prevent this from happening?

I use Nic Lodge's suggestion. Depends on how much fondant you want to color, but let's say for 1 lb. Pinch of a piece of white fondant about the size of a ping pong ball, color it with gel, paste or liquid color a shade or two darker than what you want for the final color. Knead the color well into the ball, then knead the ball into the white fondant until well combined.

I make my own MMF and if I'm making a large amount of the same color I add the coloring before adding the powdered sugar and that way it's really easy to mix in. Otherwise doing it when the fondant is ready made it's just a case of kneading and kneading until the color is even. I just looked at your cake and I actually like the swirly effect of the green even if it wasn't exactly what you'd hoped for!

i think you just needed to knead it more. knead knead knead till it all blends in.

HTH