Sorry, I was thinking Gen 4. In Gen 4 you could save in front of the guy and the nature/ability/gender and I think shininess were set, allowing you to SR for good IVs once you got what you wanted.
In Gen 5, nothing was set so you couldn't SR like that anymore.
Clearly I was tired >.<
Yeah, gen 4 was much cooler in that regard as you could get RNGed looking IVs with just hours of soft resetting. Lock in the nature, ability and shinines and SR for the IVs. Too bad gen 5 did away with that <.<
So, just to confirm:
After saving my game, if I were to hatch a shiny on say, egg number 50, I could turn my game off (as I previously saved), turn the game back on, go straight to egg 50 and hatch it and it's GUARANTEED to be shiny?
Yes that is correct. If your game produces a shiny egg, it will always hatch shiny. So, if as you said, egg 50 is shiny, you can turn off you game, pick up egg 50, and hatch it where every you want. I RNGed about 30 eggs for someone a year or so ago, and they hatch shiny on my game, but he wants them in eggs so they are still in the eggs. Regardless, if i were to hatch one, or even all 30, they would still be shiny, and yes, i hatched them all already just to verify they were right, then turned the game off without saving. I have also hatched themn all again since (because i forgot which one was which and what the exact details were) and they were indeed still shiny. Don't worry, you will not lose a shiny as long as you save before it hatches. Well, if you trade that egg away it probably won't be shiny on the next game, but that isn't at issue here.
It can be done, as the shiny factor of the egg is determined as you get it, not when you hatch it.
Unless your talking about gen 4, like Dangertrout pointed out earlier, that is incorrect. The game does not check the PID against the ID/SID combo until the egg hatched. Even in gen 4, if you trade the egg to a different game, the egg will not hatch shiny on the other game. The reason you can be certain that once an egg hatched shiny on your game it will always hatch shiny is because the PID is set once the egg is collected. As long as the PID, ID, and SID don't change, the egg will always be shiny.