It probably comes as no surprise that colours give your picture more impact. The tricky part is: how to use colours?
So, speaking of colours: there are three categories:
Secondary colours: green, orange and purple
Tertiary colours: yellow-orange, red-orange, red-purple, blue-purple, blue-green & yellow-green
One way to give your pictures more colour depth, is to use the colour wheel. A colour circle - based on red, yellow and blue - is traditional in the field of art. Sir Isaac Newton developed the first circular diagram of colours in 1666. The idea behind the colour wheel is that complemantary colours give a strong image by using both warm and cold colours.
Complementary colors are opposite each other on the colour wheel. So for example you can use red and green or blue and orange. By using those opposite colours you add more depth to you photo.
There is a great blog which explains the use of colours in the foreground and background of your photo. You can also watch this video on Youtube from Serge Ramelli. He postprocessed a great sunset image using the technique of colours to add depth to his picture.
Another way to add colour to your image, is to use colours in the same range. If this aroused your interested, make sure to check out this cool guy: Aaron Nace from Phlearn. He has a really quick and interesting explanation on the use of the colour wheel in this video. More interesting stuff can be found on his site.
Now go out there and be colourfull!