I recently went through my CSS file and switched all my six-digit hexadecimal codes to simple three-digit codes (for example, my #FDFEFF got shortened to #FFF).
It renders pretty much the exact same color as before, and it seems to me that the in-between parts are fairly useless and removing them saved me an entire 300 bytes in my CSS file.
Does it matter which version you use? I rarely ever run across websites that use only the three-digit codes (or I guess I just never run across ones that do). Is it still perfectly valid to use three-digit codes over six-digit codes, or are we supposed to use the full six-digit codes?
The three-digit codes are a shorthand, and #123 is the same as #112233. In the example you give, you've (effectively) swapped #FDFEFF for #FFFFFF, which is close to the original colour, but obviously not exact.
It doesn't "matter" which version you use, as such, but three-digit colour codes mean you have a little less choice in shades. If you feel that saving 300 bytes is worth that, then go ahead and use the three-digit codes, but unless you're designing for a low-bandwidth situation those 300 bytes won't really save you all that much.