iterm vs. zsh features

You are comparing apples to oranges.

iTerm is an application that shows a terminal window. Inside that window a shell will be run. There are many different shells. One of those shells is zsh

When it comes to coloring, iTerm allows coloring in the shell. The shell itself must also allow coloring, most do by default these days. The shell could be configured to do coloring e.g. when running built in commands like ls (configured by the CLICOLROS and LSCOLORS variable).

By "shortcuts" I think you mean "aliases". Shells allow aliases to be set up. None are normally set up by default, but if you use some standard library for zsh like oh-my-zsh you will get many defaults.

Iterm does not give you shortcuts like gco. It is a terminal emulator. It is the shell ( like zsh or bash ) that enables you to define aliases. All popular shells nowadays provide support for such aliases. But zsh along with oh-my-zsh do provide a lot of nifty features for git out of the box with just a few lines of extra configuration.

Adding to @Klas's answer:

If your terminal application doesn't support coloring, any colors supported by your shell( zsh or bash or csh ) wouldn't show up. That being said, all basic terminal applications nowadays support basic coloring. So if it is just color support you are looking for, you don't need Iterm ( assuming you are using the default Terminal.app ).

But there are a lot of really good features of Iterm which you might use later down the line. My favourite feature provided by Iterm is Shell integration and tmux support.