I started collecting B-Bags in 2002/2003 ... and I have to say that (IMO) the hardest finds are the old Yellows (Marigold & Mustard). It's especially hard to find them in good condition, since the lighter color bags really show the wear more than the darker color bags.
The same holds true for Anis, Seafoam & Pistachio ... a good condition one will go for big money on eBay (or a consignment store - unless they don't know what they've got).
That being said ... what I find really odd is the minute someone seems to mention that they are looking for an older Bag/Color (2005 for instance) ... all of a sudden a bunch of them appear on eBay for beaucoup money! Yes ... Balenciaga seems to be (finally) re-introducing the colors of yore, but they really aren't the same. The '07 Jaune (French for Yellow) ... NOT EVEN CLOSE!!! Frankly, it looked more like my '03 Mustard! If you're buying B-Bags for resale only, just remember ... that the market can be very strange ... and you may not always make the profit that you think you will. I know plenty of people who have lost money on eBay as well!