My printer was printing out masked texted with a black background. This was actually happening to any computer that printed to the printer from AutoCad that was on our network. This is what I did to fix it and it has worked perfectly.
I beleive this works because autocad is looking to at the masked as a white image which it wants to print out black. So if you switch it to custom...it automatically won't switch it.
Hope it works for you too!