I thought that the letter codes were the colors at first as well, but I put in an IMEI for a violet one, and it came out as a C800DG. I did get a hold of an LG Eclypse (C800G) and it is the exact same as my own C800DG, except it has a search button instead of the Genius button used by T-Mobile, so I do think it's more than just a color code, but also software. I think there may have been slight hardware differences within the revisions as well. I don't know.
With my old P509TN Optimus T Titanium, the TN was the color code on the sticker, but it did not show up in the XML data from LG. Whereas with my C800, the sticker ONLY says C800, and the XML data said DG.