Can't get tkinter button to change color based on changes in data

[BashBedlam]

Here is one way to go about it.

import tkinter as tk
 
rowspacer=40
colspacer=40
 
class demoForm():
	def __init__ (self) :
		self.root = tk.Tk ()
		self.fName = tk.StringVar ()
		self.fName.trace_add ('write', self.changeButton)

	def fillDemoForm(self):
		self.btnSave = tk.Button(text="Save")
		self.btnSave.place(x=colspacer*6, y=rowspacer*1)
 
		blFname= tk.Label(text='fName').place(x=10, y=10)
		fldFname = tk.Entry(self.root,textvariable=self.fName )
		fldFname.place(x=60, y= 10)
		fldFname.delete(0,tk.END)
		fldFname.insert(0,self.fName)
 
		self.root.geometry('325x250')
		self.root.configure(background = "lightgray")
 
		self.btnSave.config (bg = 'lightgray') 
		self.root.mainloop()

	def changeButton(self, *args):
		self.btnSave.config (bg="red")
 
test = demoForm ()
test.fillDemoForm()