Yellow Foods! Colorful Cooking Without Artificial Dyes.

Like red plant foods, yellow plants get their color from the whole gamut of molecules: carotenoids, anthocyanins & betalains. But the backbone of most yellow foods (like with orange foods) has to be the carotenoids. Like their orange siblings, peppers, tomatoes, citrus and carrots that are yellow are usually vibrant, and won't easily bleed or losing their color during cooking. But there are a few very notable exceptions. Only a handful of food plants get their color from betalains-- beets, chard and cactus fruit among them. Beets and chard can both come in brilliant reds or electric yellows. These betalains are strong, stable pigments that do tend to bleed and stain (though not as much as the red varieties.)  Yellow foods that get their pigment from anthocyanins (corn, cauliflower, wheat) tend to have a more pale yellow. These yellows tend to be a bit more stable than red and purple anthocyanins, but they aren't as brilliant. Saffron and other flowers are the big exception. An aromatic and colorful spice, saffron gladly bleeds it's yellow anthocyanins all over everything else. Finally the most common and potent natural yellow dye is turmeric. Turmeric's coloring compound is called curcumin, and is used to color commercial mustards. Here's the scoop on some specific yellow plant foods along with a few ideas for adding these fantastic yellow foods to your plate. As with the red foods, I've divided the yellows into fat soluble and water soluble pigments-- with fat soluble you don't have to worry about bleeding, with water soluble the color may vary or bleed depending on the preparation.

Fat-Soluble
Lemons/Citrus
The skin of lemons has a gorgeous yellow. Strips of lemon zest can add a lemony flavor and color to many dishes (a gorgeous contrast to dark green salad leaves). If you want a milder flavor, blanch the zest in boiling water first and some of the flavor will be diluted, but the color will remain brilliant. Or try saving your citrus peels to make candied citrus peel.
Passion Fruit
Lemon may be the classic yellow fruit, but the juice of a passion fruit is more yellow than that of a lemon. Here in the northeast US, whole passion fruits are always very expensive, and typically not high quality. Look for frozen passion fruit puree in specialty stores. A little passion fruit in an ice cream, sorbet or mixed drink goes a long way. Try substituting passion fruit puree in your lemon recipes (but decrease the sugar slightly, as passion fruit is not quite as acidic).
Carrots, Peppers & Tomatoes
Like the red and orange varieties, yellow peppers and tomatoes have a very stable color. Mix and match with other colors, grill them, roast them, they'll keep their bright, vivid hues.

Water-Soluble

Saffron
Saffron has the dubious distinction of being the most expensive spice. The the aromatic orange tendrils have to be painstakingly harvested from each purple saffron flower. Soak saffron before adding it to any dish so that the color and flavor distribute evenly throughout. Marigold and turmeric are sometimes used as a "poor man's" saffron. The flavor is quite different, but the distinctive color is there.
Marigold, Calendula, Dandelions & Nasturtium
Saffron may be the most prized, but it's certainly not the only edible flower that can bring yellow to your dish. Dandelions are very easy to come by (just make sure that you know that dangerous chemicals weren't used where you're harvesting). Likewise marigold, calendula and nasturtium have beautiful yellow petals that can make a striking garnish to cold dishes. Again, it is always advisable to check the source of the flowers you are getting to make sure that they are not using pesticides that are not safe for food plants. Opt for organic flowers if possible.
Cauliflower
Cauliflower varieties come in more than just plain old white, look for purple, yellow and wild looking green romanesco cauliflowers. The color of these varieties is somewhat fragile and sensitive to heat, so if you want a bright color steam them until they are al dente, then immediately shock them in a large pot of ice water.
Swiss Chard
One of the few foods to get it's pigment from betalains, Swiss chard can come in the standard (red) color... or you can get rainbow chard which has white, electric yellow and bright magenta too. These brilliant vegetables are their own colorful wonder- choosing the rainbow variety is a perfect way to add incredible color to a meal.
Beets
Beets (another betalain plant) are classically red, but the golden variety is fairly common too. The golden pigment is much less strong than in red beets, which is why many people prefer preparing golden beets than their messy, bloody red cousins. Though golden beets are beautiful, the gold colored betalains do not have the same antioxidant properties as do red betalains. So golden beets play better with their neighbors, but red beets are a little more virtuous in the nutrition world.
Corn & Corn Flour
Most varieties of corn sport the pale anthocyanin yellow. Like other fruits that get their color from anthocyanins, be careful not to overcook fresh corn. Corn flour is usually a reliable source of yellow in all baked goods. Cooked golden polenta makes a delicious (and thrifty!) base to many a plate while adding a beautiful yellow backdrop to your meal.
Semolina
Golden semolina flour is not from corn, though it looks very similar to finely ground corn flour. Experiment with substituting some semolina for regular flour in baked goods to give them a golden hue and distinctive flavor. (Don't substitute more than half of the total quantity of flour).
Olive Oil
They may look green in the bottle, but most olive oils look decidedly yellow when they are drizzled on a plate. Choose a white plate to serve your dish on and that finishing drizzle of olive oil will provide a golden accent as well as delicious flavor.
Turmeric
Turmeric deserves to be in a category of its own. This root has a yellow pigment that just won't quit. In fact, you can use turmeric to dye fabric or yarn. Like beets, turmeric will turn everything it can yellow. So you can use a turmeric concentrate as a natural food dye. (Commercial mustard is colored with curcumin, turmeric's natural pigment). If you don't want turmeric's flavor, but you do want the color, leave powdered turmeric out to stale. The flavor will degenerate rather quickly, but the color will still be vivid. Most Americans know turmeric as a dried powder, but like ginger, turmeric can also be bought fresh and grated for cooking. Also like ginger the flavor of fresh turmeric is much brighter and has a citrusy undertone.