Red, Yellow, Orange

Contrary to the popular idea that red and yellow are colors signifying danger because poisonous animals display these colors as a warning, another theory says that "mammals developed the ability to distinguish between red, yellow and orange in order to identify ripe fruit." according to an interesting article, Red and Yellow Kills a Fellow.

Although it is popular today to see McDonalds and other fast food joints that advertise their food with red and yellow as a metaphor for the supposed dangers lurking within fast food, in reality fast food restaurants were safer places to eat than choosing from unfamiliar eateries and diners, which used to be referred to as "ptomaine Tommie's" prior to the emergence of clean, safe fast food places like White Castle or McDonalds.

There must some other explanation for the ubiquitous red and yellow cardboard french fry, popcorn boxes and "golden arches" and red and yellow sign, and this sounds like a reasonable one. It makes sense that humans would be attracted to the reds and yellows of fruits. We don't run screaming from the table when presented with pasta and tomato sauce or yellow squash.

The vibrancy of red and yellow is probably the real reason we are attracted to the fast food signs.

Isolated Color

When an element of color is seen against a larger background of neutral color, the effect is called isolated color. This is a useful technique for isolating the subject and creating interest. The eye is drawn to the subject by the isolated element of color in the same way it is drawn to a highlight. On a gray day, white can substitute for color against a subdued color background (white against desaturated color).

Although the image here is an example of isolated color, it is also an example of color harmony, since the browns and tans of the concrete, composed of stone aggregate, harmonizes (shares similar tones) with the leaf. The colorful element should fill a significant percentage of the frame to be effective.