The cake below, the top is a light blue and the bottom seems to be greenish yellow?  Wondering what colors I should start with to achieve the same colors? I have electric green from americolor if that helps.

AI hope this helps and isn't too basic. Start with a very small piece of white fondant and add a tiny amount of Americolor royal blue. Mix it very good. Add more white and see if it looks like you want. For the greenish yellow I would use your electric green and do it the same way. If you make a small amount of a color, you really have not wasted anything and it is easy to adjust to find the right blend.

Some of the best light colors are made with what is typically a darker color, like Forrest green, dark brown, gold, teal, etc.

Thanks! You were right it was those color just lightly added.  I kind of wished I added a bit more green for the bottom tier but was too afraid of it turning dark.  It doesnt look that yellow in person, can tell its green.

On a brighter note my first time making a fondant character :D

You are welcome!!!!!  I am glad to help.  I am kind of a color freak.  When I went to art school a few hundred years ago, they used to have a system for us to look at each color and break it down, by the color, tint and shade.  I look at colors and I become a paint chip machine. 

It also matters what kind of fondant you use, as it has color in it too that may not be exactly white.  Plus some colors may get darker.    

Nice first monkey!!!!   Your a doing great!!!!