Summary:
Hi,
This PR Solves this issue #3083.
This PR solves the problem of default color on TabBar being always grey. Which looks great if the barTintColor is unchanged. However if we set the barTintColor to something else (like blue in example) text and icons become quite unreadable.

Commit (c206417) - Enable setting color of unselected tabs
Solves this issue with a prop (unselectedTintColor) on TabBarIOS to which you just pass a color like you can for barTintColor and tintColor.
This leaves us with a result that is on second picture. Notice the color of text on tabs.

Or change it to yellow for demonstrating purposes
 I'm working a Win8.1 machine and couldn't build the react-native using the github repo. As soon as I figure that out, I'll probably figure how to run the tests and how to add some for this feature.
2) It's my first pull request. Be gentle :)
Closes https://github.com/facebook/react-native/pull/4866
Reviewed By: bestander, svcscm
Differential Revision: D2783672
Pulled By: nicklockwood
fb-gh-sync-id: ca22aa3c0999188075681b5d20fff0631496e238