Who you add to your team will have a direct impact on the skills you'll have available in a fight. Yellow indicates regular attacks that can be strung together for combos, Red is for strong attacks that are slow to use but can't be blocked, and Blue marks defensive skills. Keep this in mind if you find yourself getting K.O'ed often. The health totals of all three of your chosen team members are all added to your overall health total. Most Rangers have both a Leader and an Assist version, but they aren't interchangeable. Leaders are the only Rangers you can control directly.
The Rangers you decide to bring into battle can make a pretty big difference.