I did a two tiered fondant covered round cake and top tier white and bottom tier green. The top tier turned out good but the bottom tier cracked in several spots. I put both of the cakes in the freezer to harden the buttercream before I put the fondant on, is it possible that the cake was too cool and that caused it to crack? I used the same fondant on both the only difference being that the bottom was colored green.

What brand or recipe of fondant did you use? Sometimes just coloring the fondant will make it more prone to cracking, especially darker colors.

you can add shortening to fondant to make it softer and easier to cover the cakes. helps the cracking issues. the amount of color you add also effects the consistency, so the darker the color, the more coloring you have to add, it makes the fondant loose it's stability and it can crack.