Expected identifier, got '='?

You're actually getting this error because you forgot to close StringValue.Value by adding two quotations after to signal it is closed.

Even after that, another error appears. When calling the Color3 Value, you still must act as if it is a property. By this I mean you must include the Color3.new string before you add the numbers, it should work after this revision

Here is your revised code:

local chatstorage               = game.Workspace.ChatHistory
game.Players.PlayerAdded:connect(function(plr)
    plr.Chatted:connect(function(ChatSender)
        local StringValue       = Instance.new("StringValue", chatstorage)
        local ColorVal          = Instance.new("Color3Value", StringValue)
        if plr.Name == "LetterSlayer" then
            StringValue.Value    = plr.Name.." : "..ChatSender..""
            ColorVal.Value 		 = Color3.new(20/255, 20/255, 20/255)  --Error
        else
            StringValue.Value   = plr.Name.." : "..ChatSender..""
            ColorVal.Value      = Color3.new(255/255, 255/255, 255/255) --Error

        end
    end)
end)