Distinguish apps in OSX task switcher
Make sure you have two separate Google Chrome binaries (i.e. two .app's). Then use Finder and right-click and select Get Info for one of those binaries. The original app icon is displayed in the top left corner of the window. Drag any other icon file on top of it, and the icon of the program will change.

You'll probably have to restart Chrome in order for this to show up in the app switcher.

An even easier way would be to use Chrome Canary for browsing and ordinary Chrome for Selenium (if that is acceptable for your use case).