Green cheeks tend to have a raspy/gritty voice, BUT mine has learned a variety of words. He knows;
Allo (Hello in French)
Fais Caca (Go poop in French)
Beau tit bebe (Beautiful little baby in French)
Thank you
Oui (Yes in French)
Yum Yum Yum
Be Good
Whatchu doing?
Love you
Chubby (his name)
He uses many of these words in context. For instance, he says thank you when I give him something he wants. He says "yum yum yum" when he wants supper. He will also say "watchu doing" when he is curious and "fais caca" before he does his poops.

He also mimics the beeping of the microwave and the creaking sound the front door makes when someone opens it. AND he picked up our laughs, so when we laugh he mocks us.

As far as trick training, it does take some time with him because he is super stubborn and energetic, so it's hard to get him to sit still for training. I've taught him to "shake hands" and "turn around" so far though. I've had to get a lock for his cage door because the little bugger figured out how to open it on his own.
So yes, I would say they are very smart, especially for a small parrot. But that intelligence comes coupled with high levels of energy, mischievousness, and sometimes loud screaming. He needs to be watched at ALL times when out of the cage, otherwise he gets into stuff he shouldn't.

I'm not sure if they can learn to recognize numbers and colors. I'm wondering if anyone has ever really tried to teach them these concepts? It would be an interesting research project to undertake

"She was not quite what you would call refined. She was not quite what you would call unrefined. She was the kind of person that keeps a parrot." - Mark Twain