How to create a new color with tkinter

Question

So I'm trying to make new color called rainbow so that I can make multi-colored fireworks. This is my code

colors=["red","green","blue","yellow","white","orange","purple"]
class Explosion:
        def __init__(self,canvas,radius=80,color="rainbow",l=15,d=[],active=False):
                self.canvas=canvas
                self.radius=radius
                if color=="rainbow":
                        self.color=random.choice(colors)
                else:
                        self.color=color
                self.l=l
                self.d=d
                self.__active=active

Also, if the color is not rainbow I want it to just be that color. "rainbow" is not recognized as a color, but I'm not sure how to get it to work. This code is simplified because this is the only part that's not working.