How to change color of individual pebble action bar slot?



I am attempting to clone the official pebble stopwatch app as an exercise and am trying to understand how the developers were able to get the segment of the action bar corresponding to the ‘select’ slot to invert colors while clicked. (screenshot included below) The official documentation for the action bar layer has several options for animations but does not seem to include any color control. Additionally, there does seem to be a way to set the background color of the entire action bar via void action_bar_layer_set_background_color(ActionBarLayer * action_bar, GColor background_color) but no obvious way to change the color of the three individual slots.

What is the best/standard way of achieving this result?