Script Error "String Expected" Help?

I have no idea how I'll make my script work. It changes material. I would love a detailed explanation because I sometime have problems with Strings.

wait(1)
for i,v in pairs(script.Parent.Buttons:GetChildren()) do
    if v:FindFirstChild("ClickDetector") then
        v.ClickDetector.MouseClick:connect(function()
            script.Parent.Color.Material = v.Material
            script.Parent.Color2.Material = v.Material
            script.Parent.Parent.Parent.DropMaterial.Value = v.Material --Problem here
        end)
    end
end

DropMaterial is a StringValue