Hello,
color change with Treeview does not work in Python 3.7.3,
but it works with Python 3.7.2.

Test code:
# -*- coding: utf-8 -*-
import tkinter
from tkinter import ttk
root = tkinter.Tk ()
style = ttk.Style (root)
style.configure ("Treeview", foreground="yellow", background="grey", fieldbackground="green")
tree = ttk.Treeview (root, columns=('Data'))
tree.heading ('#0', text='Item')
tree.heading ('#1', text='Data')
tree.insert ("", "end", text="Item_0", values=100, tags="A")
tree.insert ("", "end", text="Item_1", values=200, tags="B")
tree.insert ("", "end", text="Item_2", values=300, tags="C")
tree.tag_configure ("A", foreground="black") #Py 3.7.3: no effect
tree.tag_configure ("B", foreground="red")
tree.pack ()
root.mainloop ()