# Tk_entry_instant_relay.py

# Tk_entry_instant_relay.py

import tkinter

root = tkinter.Tk()
myvar = tkinter.StringVar()
myvar.set('')
mywidget = tkinter.Entry(root,textvariable=myvar,width=10)
mywidget.pack()

def instant_relay(*t):
    t = myvar.get()
    color = 'red blue cyan lightgreen yellow'.split()[len(t)%5]
    mywidget.config(bg=color)
    mywidget.update_idletasks()

myvar.trace('w',instant_relay)

root.mainloop()