Nobody asked yet, but it occurred to me to ask what temps the OP was soldering at? Unless I'm desoldering for salvage, I usually like to work at the lower temps. I was just thinking that with the limited heat transfer from the 'useless' conical tips, one might be forced into setting higher temps just to get the solder flowing, thus causing more discoloration. The solution may just be to get a chisel tip and lower the temps.