You could make custom emotions for each, with IDs like 
%bd:r%, 
%bd:w%, 
%bd:b% and 
%bd:y%, and then make a script that sends 
%bd:?%, where 
? is replaced by a random letter (maybe using Math.random() and an array).
js code:
var Letters = ["r", "w", "b", "y"];
function OnChatWndSendMessage(PlusWnd, Message)
{
    if (Message.toLowerCase() === "/beaddraw")
    {
        return "%bd:" + Letters[Math.floor(Math.random() * 3)] + "%";
    }
}