I have tried -
function onClicked() script.Parent.BackgroundColor3 = Color3.new(121, 121, 121) script.Parent.MouseButton1Click:connect(onClicked)
And the color ended up being - (30855, 30855, 30855)
You must diving by 255 to get a 0 - 1 value
function onClicked() script.Parent.BackgroundColor3 = Color3.new(121/255, 121/255, 121/255) end script.Parent.MouseButton1Click:connect(onClicked)