Just Bento

I try to vary the colors I use. But my only goal behind that was to get a variety of fruits and vegetables and the different vitamins and minerals that go along with them and to kind of make things less blah (eating plain green lettuce everyday just does not cut it for me).

I think there was a commercial here in the US promoting eating the rainbow everyday, but I guess anything to help Americans eat better.

Hi ginger. The 'five' rules that niceties posted about are really guidelines for Japanese haute cuisine, or kaiseki. We might aspire to those rules in theory, but in practice, in everyday cooking, it's rather difficult! What I do try for is a balance of flavors - say, if there's a rich or oily dish (steak) have something sharp and refreshing to go with it (salad), and so on. For bentos, if the main protein is say, fried shrimp having a little bit of sweet and a bit of sour (e.g., some fruit, some pickles) can help to balance the meal. Trying for a variety of colors can also help to balance a meal or bento, especially since the most colorful foods are fruit and vegetables. An all-brown-beige kind of meal is not only visually unappetizing, it is usually not that balanced. (Think of a hamburger with fries for example, with only a bit of green from the lettuce for color).

Similar principles are present in Chinese and Korean cooking as they've grown out of traditional medicine, but I don't know how prevalent they are in everyday contemporary cooking. Ayurvedic cooking, which is practiced in South Asia, uses similar, though not identical, principles as well. I once asked my acupuncturist, who is trained in traditional eastern medicine, if traditional Chinese medicine grew out of ayurveda, as Chinese Buddhism came from Indian Buddhism. But she wasn't sure.
One fun way to observe these principles is in the popular Korean drama "Da Janggeum", about a female cook and doctor (based on a real person) in the court of a Korean emperor long ago. It shows how closely cooks and doctors worked together, and how food was considered a form of medicine.
I try to balance the five tastes and styles in my cooking (I'm not remotely Asian; I'm born and bred in Atlanta, GA, USA!) and usually fail, but I've found that the principle of five flavors has worked its way into my subconscious. I can't even have a @#&! packet of instant noodles without throwing 3 kinds of veg on top. I feel depressed if my food is all one color.