I heard the expression "green horses" but I don't ...

This metaphor relates to the idea that a young tree is often called a green sapling. Young horses in training are called “green”. Whereas experienced horses are often referred to as “made”.

"Green horse" is also a phrase you want to avoid when you're looking for a horse to buy, lol.

Horses really aren't the color green, it just means they either have little to no training or they are not broke or never been ridden. right now i have a 3 yr old horse she is considered Green. The rider can also be called green if he or she is inexperienced. Hopefully you understand.

Jess 

Don't worry! Only nature decides the color of a horse. Its just a phrase. The "green saplings" is a perfect anaology. Just as many fruit are considered "green" when they're not ripe yet, just so a horse which is not considered trained is refered to as "green."

Green broke means little experience undersaddle. Not for someone new to horses.