The color of an object depends on wavelengths of color being either absorbed or reflected. For example, white reflects all color wavelengths, while oranges are orange because they reflect the orange color wavelength in natural light, called white light. Colors relate to heat because colors that absorb more light wavelengths, typically darker colors, turn that light into energy in the form of heat.
Black absorbs all color wavelengths. This is why the color black is described as having no color. This also means that a black object must turn the absorbed light into energy. Black absorbs the most light of any color and creates the most heat. As such, it is wise to avoid black clothing on sunny days, not because it attracts heat, but because it absorbs light that is then turned into heat.
Blue objects attract varying amounts of light, making some shades hotter to wear than other shades. Light blues, such as aqua or robin's egg, reflect most light wavelengths and therefore create less heat. Dark shades, such as royal blue, absorb more light. This why a dark blue car will feel hotter inside than a white car on a summer day. Navy is almost as dark as black, making it the second hottest-feeling color.
Purple is similar to blue in that both colors have shades that vary widely from light to dark. Lilacs and lavenders reflect most color wavelengths and produce less energy, so they remain cooler. Violet and magenta are in mid-range and darker shades such as wine or eggplant are near black on the color spectrum. Eggplant or aubergine shades of purple are only slightly lighter than navy, making them the third hottest-feeling color. Avoid these shades for curtains or paint in a room with a lot of windows because the room will be very hot during the summer.
Green is usually a mid-range color on the spectrum, absorbing a medium amount of light. Lighter greens will feel cooler. This is why pastel shades of green and other colors are popular for summer clothing. Dark green shades such as hunter or forest green absorb much more light and create a lot of heat. These colors are more suitable in fall and winter decor and clothing and are not ideal in the summer months.