Flashing Gui Script?

Your problem was here Script.Parent.BackgroundColor3 = 250, 0, 0 firstly you need to add parentheses where the numbers are and secondly you forgot this BackgroundColor3 = Color3.new it's not BackgroundColor3 = 250, 0, 0 here is the fixed script

i also made frame = script.Parent to make it easier to understand

frame = script.Parent
while true do 
    wait()
    frame.BackgroundColor3 = Color3.new(255, 255, 255)
     wait(.1)
     frame.BackgroundColor3 = Color3.new(0, 0, 0)

end

Color3 can only take the values between 0 and 1, not 0-255. Divide numbers by 255 like this:

while true do 
Script.Parent.BackgroundColor3 = (250/255, 0, 0)
wait(.1) 
Script.Parent.BackgroundColor3 = (0, 0, 0)

end

You need to make it have a lower s. Eg;

while true do 
script.Parent.BackgroundColor3 = Color3.new(255, 255, 255)
wait(.1) 
script.Parent.BackgroundColor3 = Color3.new(0, 0, 0)

end

There you go