Unicorn sushi or sushi rainbow
First things first. Four things to keep in mind:
1. Natural colorants often lend a more demure hue than their petroleum-laden cousins. This is merely due to ratios : a concentrated store-bought food coloring requires only a few drops to add color and thus doesn't change the texture of the food by adding vast amounts of liquid. By comparison, it's difficult to get a true “red” from beet juice rather than just pink, not because the beet juice isn't red enough, but because most recipes can't stand up to the amount of liquid required to obtain a true red.
HOWEVER, you CAN make concentrated natural food dyes. See below to learn how.
On the other hand, just a few drops of beet juice will create an absolutely lovely pink in frostings and ice creams without adding even a hint of beet flavor. (See the list below for a number of gorgeous examples….)
2. Since these colorants are foods themselves, remember that strongly colored foods also tend to be strongly flavored foods, so try – as much as possible – to avoid using large quantities of any of these (a small amount of cinnamon, for example, might make a frosting heavenly, but a large amount would render it inedible).
Also keep your final flavor in mind so that the colorants don't overwhelm it. I'm assuming you don't necessarily want paprika spiked buttercream or parsley ice cream! Again, small amounts of those colorants won't even be noticeable, but large amounts most certainly will.
3. Vegetable powders are great ways to add vivid colors without adding excess liquid, so if you're wanting a deep red, for example, use beet powder rather than beet juice. Another alternative is to reduce your liquid to a concentrated syrup, so see even farther below for a two quick tutorials on how to make vegetable powders and how to make concentrated homemade food coloring, just like you'd buy in a store.
4. Vegetables-based dyes can turn brown when baked, when otherwise put in an alkaline environment, or when sufficiently oxidized. Thus, vegetable dyes WILL NOT WORK when baked unless the batter or dough includes sufficient acid medium, such as buttermilk, lemon juice, or vinegar. I highly recommend stirring lemon juice into your vegetable juice (e.g. parsley juice, carrot juice, or beet juice) in at least a 1:6 ratio if making a rainbow cake or red velvet cupcake
You can also help keep the dyes at their vibrant-y best by cooking the vegetables ahead and blending them into a purée rather than using the raw juice.
Making your own homemade food coloring is easier than you think!
Natural Food Dyes / Homemade Food Coloring
Start with a small pinch or a few drops of these colorants and add more according to your desired shade.
RED:
pure beet juice
beet powder
pure pomegranate juice
red raspberry purée, strained to remove seeds
PINK:
pure beet juice
pure cranberry juice
pure raspberry juice
ORANGE:
pure carrot juice
carrot powder
paprika
YELLOW
fresh turmeric juice
ground turmeric
saffron
GREEN:
liquid chlorophyll (see where to buy liquid chlorophyll)
matcha powder (see where to buy matcha)
spirulina powder (see where to buy spirulina powder) – use sparingly
parsley juice
wheatgrass juice
spinach juice
spinach powder
parsley powder
BLUE:
(FYI: Blues and purples are notorious for being the most difficult dyes to produce, either naturally or in a lab, so it can be tricky to get the right hue. Blue butterfly pea flowers, interestingly enough, are by far the most reliable way to get a beautiful blue or violet. Purple sweet potatoes lend a lovely deeper purple.)
blue butterfly pea flowers (see where to buy butterfly pea flowers)
red cabbage juice
stir a bit of baking soda in with red cabbage juice or a purple dye for a brighter blue
PURPLE:
purple sweet potatoes
blue butterfly pea flowers + an acid, such as lemon juice
pure blueberry juice
purple grape juice, concentrated
TAN:
bentonite clay powder (bentonite is an edible clay) – use very sparingly
a very small amount of cocoa powder
BROWN:
cocoa powder
pure espresso
instant coffee granules
heavily steeped black tea
cinnamon
BLACK:
black cocoa powder (see where to buy black cocoa)
activated charcoal powder (yes, it’s safe to eat and even beneficial!)
squid ink (see where to buy squid ink)
And of course, if you can't get the color you want from making your own homemade natural food dyes, you can always buy a vegetable-based food coloring, but they're pricey: See Vegetable-based food coloring.