I used to think an apple was just an apple. Boy, was I wrong. Certain varieties of apples have 10 times the vitamins, minerals and other nutrients of lesser apples.

I used to think an apple was just an apple. Boy, was I wrong. Certain varieties of apples have 10 times the vitamins, minerals and other nutrients of lesser apples. The same is true for other fruits and vegetables. Just ask Jo Robinson, author of Eating on the Wild Side: The Missing Link to Optimum Health. She surveyed the scientific literature of studies that prove certain fruits and vegetables are better for us.

Why do I bring this up? Because the cost of food overall has risen 21 percent since 2006, according to the USDA. That's just the average. Between April 2013 and April 2014, the cost of fruit alone rose more than six percent. Beef and pork rose more than 10 percent during that year, and prices are set to rise even more soon, thanks to California's ongoing drought. Food prices are up, and your paycheck probably isn't keeping up.

So, when you head to the grocery store, make sure you're getting maximum nutrition for every dollar you spend. One of the easiest ways to do that is to shift your shopping in the produce aisle to more nutrient-dense varieties. It will pay a dividend in better health.

What should you look for? "If it's purple, red, blue or black, eat it," Robinson said in a recent interview with NPR's Science Friday. Fruits and vegetables with those colors - including plums, berries and salad greens - contain "one of the most well researched and beneficial phytonutrients that we know - anthocyanins."

Anthocyanins are believed to reduce the risk of cancer, Alzheimer's, diabetes and cardiovascular disease. "We need a shopping list based on science," she said.

What does that shopping list look like? You can download a full shopping guide at Robinson's website, eatwild.com. In the meantime, here is a quick cheat sheet to get you started.

First, eat more artichokes, which apparently are one of the most nutritious vegetables in the store, fresh or in a jar. Eat more garlic and onions, particularly strong-tasting onions and shallots. They are nutrient powerhouses that have been shown to improve health overall. And remember those reds, purples, blues and blacks.

Salad greens: When shopping for salads, go for varieties with leaves that are very dark green, red or purple. Or opt for arugula and spinach, both of which contain more antioxidants - including lutein, which is good for eyes - than most lettuces.

Potatoes: New or fingerling potatoes have a smaller impact on blood sugar than larger baking potatoes, which is important for diabetics. Otherwise, Russet Burbank potatoes, potatoes with red skin or novelty potatoes with colored skin and colored flesh have more vitamins and minerals than traditional white and yellow potatoes. Buy organic, if possible, because potatoes absorb fungicides and pesticides.

Carrots: Any carrot is good for you, but the colorful red or purple carrots you can find at specialty and farmers' markets contain the most nutrients. When they aren't available, look for the deepest orange carrots, and buy ones with green tops still attached. They're fresher. Eat the skin, because it contains the most nutrients.

Tomatoes: They contain lycopene, an antioxidant believed to shield us from cancer and heart disease. The most nutritious tomatoes in the grocery store are actually in a can. Canned tomatoes ripen longer on the vine, enhancing flavor and nutrient density; heat applied during the canning process also makes lycopene easier to digest. When shopping for fresh, go for cherry or grape tomatoes. The smaller and redder the tomato, the better it is for you. At specialty markets, look for tomatoes streaked with blue or black.

Apples can be like biting into the alphabet, as they contain vitamins A, B, C, E, K and a host of other nutrients believed to lower cancer risk. But not all apples are created equal. In the store, opt for Gala, Fuji, Granny Smith, Honeycrisp or Red Delicious. They offer the most vitamins for your dollar. And eat the skins - they store about 50 percent of the nutrients.

-Denise Trowbridge is a self-professed money geek who writes about personal finance, banking and insurance for The Columbus Dispatch, bankrate.com and middlepathfinance.