Ye, you are right, the DS Phat sales were pretty impressive in the end of 2005 indeed, i wasnt aware of that, so thanks for mentioning this!
But after checking at
Garaph.info, i see that it was not until the DS Lite was released that the DS was selling over 100k systems a week for week after week for about 1.5 year in a row (with some expections, in the 78 weeks from the DS Lite launch, only 6 week of those weeks had DS Lite hardware sales under 100k). EDIT: These numbers are based on the Media Create numbers at Garaph.info. Based on the Famitsu numbers, only 3 weeks out of those 78 weeks were under 100k, and one of these 3 weeks were almost 100k (99,740 to be precise)
It might have been a bit of a "coincidence" (or what i shall say) that the DS Lite was released shortly after (about 3 months) the end of 2005 when the DS sold really well. If the DS Lite wasnt released, maybe the DS Phat would have sold over 100k a week for week after week as well just like the DS Lite did?
But the DS Lite color was white in general (i think so at least?), and many people bought it. I just wanted to mention that i personally dont think that a color of a gaming system matters that much, at least when the colors are either black, white or grey. I do belive that the color of a gaming system might have some affect on the sales though, but as long as a gaming system have interesting games and a good pricetag, i think that this is the most important thing to people in general, but that is just my opinion