Change the colour of your object with the fill command
blob2=win.create_rectangle(150,150,200,200, fill="red")
 In this example we have added the fill command after the coordinates to change colour.
Flashing Colours
you will need to import random at the top for this to work

col=["red","blue","green"] #create a list of colours
blob2=win.create_rectangle(150,150,200,200, fill="red") #create rectangle in blob2 variable
for i in range(60): #60 repeat loops
    win.move(blob2,0,-2) #move blob2 -2 on y axis
    changecol=random.choice(col) #make a random choice from col list and put it into changecol variable
    win.itemconfig(blob2, fill=changecol) #update blob2 with new random choice colour
    tk.update() #update the window
    time.sleep(0.05) #short time delay to slow things down

Remember this will only work if it inside a canvas with mainloop at the bottom