Since I can't post a follow-up to android developer's comment, here's an updated answer for anyone else who needs to programmatically set the selected tab indicator color:
tabLayout.setSelectedTabIndicatorColor(Color.parseColor("#FFFFFF"));
Similarly, for height:
tabLayout.setSelectedTabIndicatorHeight((int) (2 * getResources().getDisplayMetrics().density));
These methods were only recently added to revision 23.0.0 of the Support Library, which is why Soheil Setayeshi's answer uses reflection.