Why does changing a UI elements background color using color3 make it all weird?

so for some reason when I change a button's background color with

this script

local player = game.Players.LocalPlayer
local button = script.Parent

script.Parent.MouseEnter:connect(function(over)
    button.Size = UDim2.new(0, 210, 0, 50)
    button.BackgroundColor3 = Color3.new(0, 255, 0)
    button.Position = UDim2.new(0, 15, 0.6, 0)
end)

script.Parent.MouseLeave:connect(function(over)
    button.Size = UDim2.new(0, 200, 0, 50)
    button.Position = UDim2.new(0, 0, 0.6, 0)
    button.BackgroundColor3 = Color3.new(27, 42, 53)
end)

after my mouse leave event fires the button is white and has a color3 value of

6885, 10710, 13515 what causes this and how do I fix it?