I have two repositories. In one, I make changes to file ./hello.test. I commit the changes and create a patch from that commit with git format-patch -1 HE
You could create the patch using git diff and then apply it using the patch utility, which allows you to specify the file you want to apply the diff to.
For example:
cd first-repo
git diff HEAD^ -- hello.test > ~/patch_file
cd ../second-repo
patch -p1 blue/red/hi.test ~/patch_file