Simple interval timer in Python
Don't use wildcard imports

I know that most tkinter tutorials show you to do from tkinter import *, but they are wrong. PEP8 recommends not use use wildcard imports, and I think that's especially true with tkinter. 

Instead, use import tkinter as tk, and then prefix all tk classes with tk.. For example:

master = tk.Tk()
...
label1 = tk.Label(master, text="Working Time", bg='white')


You are using after incorrectly

This statement does not do what you think it does:

master.after(1000, label5.config(text=i))


The above is exactly the same as this:

result = label5.config(text=i)
master.after(1000, result)


My advice is to always have after call a function. So, write a function that takes two arguments, a label and a value, and updates the label with a value. Then, use after to call that function.

def update_label(label, value):
    label.configure(text=value)
...
master.after(1000, update_label, label5, i)


Give your widgets explicit masters / parents

Instead of this:

button1=Button(text='Window Color', command=window_color)


Do this:

button1=Button(master, text='Window Color', command=window_color)


While tkinter will use the default root window if you leave it out, PEP8 says that explicit is better than implicit. Plus, it makes it clear what your intentions are. Otherwise, someone reviewing your code might wonder if you're intentionally putting the widget in the root window or if it's a bug.

Remove useless code

I don't know what you think this is doing, but whatever that is, it's probably not doing it. You can remove this line of code:

master.after(1000, None)