Powerline Theme in OSX terminal have a strange color
There's not much you can do about it (except trying a different font). 

I had the same "issue" but I don't consider this as something bad.
Get use to it but first try a font that will match as much as possible and will reduce the difference between "rectangle" section and "triangle" section.

Check my OMZ shell script where I use bullet-train theme.


  Bullet Train theme for OMZ requires Powerline Compatible Fonts


Take a look especially at:

mkdir $TEMP_FONT_DEST
cd $TEMP_FONT_DEST
git clone https://github.com/powerline/fonts.git .
./install.sh
rm -rf $TEMP_FONT_DEST




I will upvote your question because you're new here and to wake up your creativity. Good luck with customising your shell.