Hmm... well, I just usually go to the store and grab juices in red, blue, and yellow then mix to get the colors I want. Like paint, you know? Then I sometimes get colors it'd be hard to make, like tea and such.
Good ones are Gatorade, Arizona Ice tea, FUZE, and Crystal Light from what I've seen.If you saw what I had at the MB practice, here's what it was:
Ether: Pixie sticks
Vaccine: liquid SweetTarts
Potion: Pomegranate/Cherry Crystal Light
Elixir: Light blue gatorade(blue raspberry lemonade, I believe)
Remedy: Orange Skittles
Antidote: light blue gatorade and peach Arizona
Soft: Peach Arizona and water
There were others, but I'm sure they weren't edible. xD At least, I think so. I'm too lazy to check. I'll be remaking them, adhering more to the FFIX colors in the item menu, and will throw in gysahl greens and dead peppers soon. A good way to get a nice, palish purple is to mix three parts of the lightblue Gatorade with just a bit of the cherry/pomegranate crystal light. Once I remake all of the potions with the right colors for the game, I'll list 'em all and how I made them for the peeps. ^^
EDIT
As for using candy, you can use any kind as long as the colors are all the same. E.I. no mixing green and orange Skittles. It looks dorky that way. xD But if you run out of orange skittles/runts/whatever, have the rest handy so you can just replace them all with another color. the expection was the pixie sticks; it'd take foever to fill jsut one bottle unless you're using more then one color and frankly... it looks a lot cooler mixed, liek when mixing colored sand. xD