Aragan's Blue Potion Simulator
This is the original potion recipe! Well, my original potion recipe. Being an Acolyte, I naturally tried to make a facsimile of a blue potion first!

Ingredients:
Blue-colored soda
1 tablespoon of Half-and-half cream
1~2 tablespoon(s) of Honey
15-20 drops of liquid blue food coloring

(This is for a regular glass-sized drink.)

You should probably pour the soda into the glass first. If you pour the cream in and then the soda, it'll make a hard foam that doesn't look too potiony (in my opinion). Add the cream and honey after that, and mix well—then add the blue food coloring, roughly 15 drops or so.

Why use those ingredients?:
It';s an odd list, so I might as well provide explanation:

Blue-colored Soda: Well, it's blue, but it's a tad too translucent to me. However, it's fizzly, something that reminds me of potions... and it has an odd taste.
Cream: This helps thicken the potion some, and makes it less opaque (kind of like the potions in the item description pic).
Honey: Same reasoning as the cream, and also because RO honey restores both HP and SP.
Food coloring: This helps make the concoction much more believable to me. With the amount of drops I used, my potion mix was a pretty deep blue!

Other Potions!
To make simulations of other potions, here's what you should replace the blue-colored soda and food coloring with:

Red Potion: 15-20 drops of red food coloring, red-colored soda
Orange Potion: 7~8 drops of red and yellow food coloring each, your favorite orange soda
Yellow Potion: 15-20 drops of yellow food coloring, yellow-colored soda or ginger ale
White Potion: No food coloring, clear-colored soda or ginger ale
Green Potion: 10 drops of blue and yellow food coloring each, clear-colored soda

Or, if you can find a good green-colored soda, use that and 7~8 drops of blue and yellow food coloring each.

Lo-cal Potions::
I do realize that with the honey, cream and soda, these potions are probably high in calories. Here's a few ways you could make the potions healthier:

No Soda, Just Coloring:
This is also something you can try if you can't find a good colored soda that tastes pleasant (since soft drink companies are always introducing and retiring soda flavors).

Try replacing the soda with club soda (which has no calories), or, better yet, flavored sparkling water (your choice of flavor, though I'd recommend a flavor of fruit that's a color similar to the kind of potion you're making). Whichever kind you pick, increase the amount of food coloring you use—start with half-again the usual amount and see how it works. It'll definitely have a different taste, but it'll have much less calories.

(Soy) Milk Instead of Cream:
Half and half cream fat, even if it's only a tablespoon. If you'd rather avoid having too much fat, try skim milk or soy milk. It'll be more watery than usual, of course.

More Alterations:
Of course, you don't have to stop there! There's other things you can do with the recipe.

Crushed herbs:
Try mixing in crushed or powdered herbs to further emphasize the idea that the potions are made from herbs.

It probably wouldn't be very pleasant to drink unless you used powdered herbs, though it would definitely add to the effect. Basil leaves would probably resemble RO herbs the most, but for flavor, I'd use powdered cloves. They have a really strong flavor that does well in a lot of applications, and it just might make up for a loss of flavor if you opt to use club soda rather than a colored soda for your potions.

Enjoy your faked potions!