Yours is a BTO option purchased online, hence, I believe you will have to arrange online to change it - this is easy, just call Apple Support line in your region and keep hammering away that the faults are unacceptable for a machine costing nearly US$2,500 - hopefully these issues have surfaced prior to the 14 day exchange policy, if its after this timeframe, you may have issues.
It took me three attempts to get a replacement for a 21 day old imac 27in i5 3.1Ghz.
I'm having mine replaced and updating to i7 and add more VRAM - this should arrive by late June after they have collected the original - like you I had yellow tint and what appeared a dirty screen - the mark as big as my palm - was smack bang in the middle of the display and yellow tint affected the bottom third - mine was purchased from a reseller, in future I'll only order online or purchase from an actual Apple Store - currently we do not have any in Hong Kong.
Hope this helps, but call them now, the longer you wait, the worse it will get.