[Python] How can I change my variable my_color if the function on_button_click is triggered

Im couple of days into python and I cant find the propper solution for my problem. I read global variable is not the best choice so I want to figure out the propper way. I want to change the variable my_color always if the function on_button_click is triggerd. Its giving me a new color if on_button_click is triggerd, but its not updating the my_color variable. I guess it as something to do with mutable and immutable objects maybe? How can I solve it?

class WidgetsExample(BoxLayout):
#random change in Text "color"

color_list = [[0,0,1,1],[1,0,0,1],[1,1,0,1]]
font_color = random.choice(color_list)

#color in word
colors = ["RED", "BLUE", "YELLOW", "GREEN"]
my_color = (StringProperty(""))

#sum of score
score = []
my_score = StringProperty("Punkte: ")

def on_button_click(self):
self.my_color = random.choice(self.colors)
self.score.append(5)
font_color = (random.choice(self.color_list))
print(font_color)
return self.font_color
print((sum(self.score)))
self.my_score = "Punkte: " + str(sum(self.score))

Continue reading...