Echoing from a variable in a specific part of a string

I am trying to make a command for having a nice-looking git log. Say I have the following:

git log --color --pretty=format:"%C(cyan)" --abbrev-commit

What I would like to do is to make a variable that takes %an and decides what colour it will be based on the output string. So far, I have

USER=$(if [ ! %an = "my own user name" ]
    then
        echo "%C(cyan)"
    else
        echo %C(bold blue)
    fi)

So then I put the variable into the git log such that I have

git log --color --pretty=format:"$USER" --abbrev-commit

Which should make is so that if a commit has been done by *me*, it will print my username in cyan; else, it will print the collaborator's username in bold blue. However, it doesn't. I'm just wondering if someone knows what I'm doing wrong here? I suspect it is something to do with printing the variable into an environment that is already a string, but I don't know how to fix it. Any help is appreciated!