I don't believe there is a quick answer...at least not that I'm aware of.
As sand is, essentially, silicate...as a mineral, I'd swing them closer to Metallics...If that's not enough reason, consider that silicate minerals are formed of silicon, oxygen, and one or more metals.
I, personally, would make them a Minerals dragon (and lump them in with, for example, Gemstone dragons and others)...but I think, if you had to choose between Chromatic or Metallic, they would definitely go into/under Metallic.
This is the Sandstorm Sand Dragon, correct?
There is such a large difference between that version and the Brown Dragon from Monsters of Faerun that they're definitely not the same creature. Based on how the text says they aren't Evil, but definitely not Good, I wouldn't be inclined to give them a chromatic or a metallic designation. Maybe just say a dragonwrought kobold gets 7.5 years instead of 5 for chromatic or 10 for metallic?
One thing to consider is since they did have that list on page 103 of all the true dragons to that date, it might mean that only chromatic or metallic dragonwrought kobolds get their age extended, and none of the others do. It's a bit odd.