The FS number refers to the US govt. color standard FS 595- basically, the first number indicates gloss (1), semi-gloss (2) or flat (3). The next digit refers to what actual color this is- 6 is gray, 5 is blue and so on. There is no real rhyme or reason to this part. The last three numbers refer to something called "Spectral Reflectance" or how bright or dull the color is. The lower the last three, the darker the color (in general). Good kits will use this as a paint reference because it is standard and checkable. Good paint brands (purporting to be US FS595 colors) will be mixed to this standard and have the FS number on the label. You can buy official US FS 595 color decks but they are pricey. A really great web reference for this is
- Urban Frederiksson's color pages. They explain all of this in massive detail and have paint cross-references for different brands. Another good explanation is
Don't let the gobbledegook throw ya- this is really pretty easy once you get used to it, just like juggling chainsaws!
Hope this helps