You need to do two things:
Create a new
tkinter.ttk.Style
for the button that has itsforeground
option set to red.Assign the button's
style
option to this style.
Below is a fixed version of your script:
from tkinter import *
from tkinter import ttk
def calculate(*args):
try:
valuex=int(x.get())
valuey=int(y.get())
valuez=int(z.get())
cappf.set(int(str(valuex)+str(valuey))*10**valuez)
capnf.set(int(str(valuex)+str(valuey))*10**valuez*10**-3)
capuf.set(int(str(valuex)+str(valuey))*10**valuez*10**-6)
except ValueError:
pass
root=Tk()
root.title('Capacitor Calculator')
mainframe=ttk.Frame(root, padding='10 10 10 10')
mainframe.grid(column=0, row=0, sticky=(N, W, E, S))
mainframe.columnconfigure(0, weight=1)
mainframe.rowconfigure(0, weight=1)
x=StringVar()
x_entry=ttk.Entry(mainframe, width=3, textvariable=x)
x_entry.grid(column=1, row=2, sticky=(W,E))
y=StringVar()
y_entry=ttk.Entry(mainframe, width=3, textvariable=y)
y_entry.grid(column=1, row=3, sticky=(W,E))
z=StringVar()
z_entry=ttk.Entry(mainframe, width=3, textvariable=z)
z_entry.grid(column=1, row=4, sticky=(W,E))
cappf=StringVar()
capnf=StringVar()
capuf=StringVar()
ttk.Button(mainframe, width=7, text='Calculate', command=calculate).grid(column=2, row=5, sticky=(N, E, W, S))
ttk.Label(mainframe, textvariable=cappf).grid(column=2, row=2, sticky=N)
ttk.Label(mainframe, textvariable=capnf).grid(column=2, row=3, sticky=N)
ttk.Label(mainframe, textvariable=capuf).grid(column=2, row=4, sticky=N)
ttk.Label(mainframe, text='Input:').grid(column=1, row=1, sticky=N)
ttk.Label(mainframe, text='Amount:').grid(column=2, row=1, sticky=N)
ttk.Label(mainframe, text='Units:').grid(column=3, row=1, sticky=N)
ttk.Label(mainframe, text='pf').grid(column=3, row=2, sticky=N)
ttk.Label(mainframe, text='nf').grid(column=3, row=3, sticky=N)
ttk.Label(mainframe, text='uf').grid(column=3, row=4, sticky=N)
##########################################################################################################################
ttk.Button(mainframe, text='Quit', style='red.TButton', command=root.destroy).grid(column=2, row=6, sticky=(N, E, S, W))
ttk.Style().configure('red.TButton', foreground='red', padding=6, font='Corbel 16')
##########################################################################################################################
ttk.Style().configure('TLabel', padding=6, font='Corbel 16', foreground='#0077AF')
ttk.Style().configure("TButton", padding=6, relief="flat", font='Corbel 18', foreground='#00AA11')
for child in mainframe.winfo_children(): child.grid_configure(padx=5, pady=3)
x_entry.focus()
y_entry.focus()
z_entry.focus()
root.bind('<Return>', calculate)
root.mainloop()
The stuff I changed is in the comment box.