import tkinter as tk

def on_enter(e):
    myButton['background'] = 'green'

def on_leave(e):
    myButton['background'] = 'SystemButtonFace'

root = tk.Tk()
myButton = tk.Button(root,text="Click Me")
myButton.grid()


myButton.bind("", on_enter)
myButton.bind("", on_leave)

root.mainloop()