A complimentary color scheme uses colors from the opposite sides of the color wheel. Because they are opposites, they can be used for great contrast. They can clash if used in their rawest forms of most intense value. However, when mixed, they yield the lovliest earth tones. Care must be used to prevent a "muddy" palette.
Here are examples of Red's complimentary schemes.
Red's Complimentary Color is Green
These reds have a 50% transparency of green shades.
#FF0000
#BF3030
#a60000
#FF4040
#FF7070
#00CC00
#269926
#008500
#3AE63A
#69E669