As long as you flushed your keg with CO2, and are using CO2 to dispense your beer, there is very little risk of oxidizing your beer even over long periods of time. If this was a heavily hopped beer like an IPA, I would address the concern of oxidation a bit more seriously, but the odds are hugely in favor of the fact that your beer has not been oxidized.
My guess is that the "slightly darker color" and the "different" flavor might have just been caused by the rest of your yeast flocculating out of suspension, thus resulting in clearer (but darker) beer and a less yeasty (more clean) flavor.
I know that it can be nerve-racking to serve your beer to a lot of people, but it is likely that they will love it and will praise you highly. Have faith in your system and your methods, and I'm sure everything will be fine. If all else fails, RDWHAH.
Read my homebrew blog!Fermenting:
Sea Symphony Strong AleOn tap:
Limerick Session Stout, Junior/Senior Amarillo RyePA, Ralph Partigyle Bitter, Saison de PotironOn Deck:
Yazoo Gerst Clone, Experimental Belgian Mild, Nelson Sauvin SMaSH bitter, Belgian IPA, Coffee Cream StoutRelax, don't worry, have a homebrew.