The color wheel is a convenient way to understand and remember the relationship between colors. Red, green, and blue are the additive primaries. Cyan, magenta, and yellow are the subtractive primaries. Directly across from each additive primary is its complement: red-cyan, green-magenta, and blue-yellow.
Each subtractive primary is made up of two additive primaries, but not its complement. So, if you increase the amount of a primary color in your image, you reduce the amount of its complement. For example, yellow is composed of green and red light, but there is no blue light in yellow. When adjusting yellow in Photoshop Elements, you change the color values in the blue color channel. By adding blue to your image, you subtract yellow from it.