Sorry to leave you hanging on this, for some reason my notifications on this thread got turned off.
I just did a test of this in ACF 4 and it worked as expected. I used your choices in a select field. I named the select field color
grey : Grey
black : Black
light_green : Light Green
dark_green : Dark Green
light_brown : Light Brown
dark_brown : Dark Brown
I then created a page that used the select field and selected Light Green
I then added this to my page template the_field('color');
When I viewed the page light_green as I expected.
If you are still having problems with this, please disable all plugins and switch to the default WP theme and try again. It should be working as I described and if it isn’t then you more than likely have a conflict of some kind.
