I acquired my memory about this.
I'm not expecting to find this game in Newgrounds, it's used to be popular in my neighbourhood. Good times.
And also, the rangers who you are controlling are named after his/her colour.
The bosses(except "Grey" in level 6, and the Styless Rangers) are named into "(Its corresponding animal) Drill". For instance, the boss in Level 1 is called "Turtle Drill".
The way to acquire "Grey", the 4th ranger: Click the enblem of the "Styleless Rangers", in order to turn it from red to grey.
Note that you are unable to use "Red", "Blue", and "Yellow" when you used "Grey", and vice versa.
One more thing, the "Styleless Rangers" are also known as "OO Ranger", where "OO"(Circles, more circles are OK to use) is used as a wildcard when mentioning a person, or to keep the person anonymous.