I thought I’d go into Photoshop and make an example of how you can incorporate four different colours into one theme, without it looking busy or make your eyes explode.
The way this is done is by not only making sure that all colours are compatible without one another, but also to leave the colours as accents. Much of this theoretical theme is made up of neutrals - black, grey, and white (mind you, browns are neutrals too). This is what will make sure that your design doesn’t look busy. You can use as many colours as you want this way, or as few (for example, my theme only uses orange and yellow), by simply making them accent the neutral colours you have.
Also note that there’s a fair balance between neutrals - this makes that there’s depth to the design, and makes it more interesting to look at in contrast to, say, if the grey background were to be a similar shade as the light black sidebar.
If you want to use a gradient map on the icon/sidebar graphic (see, the turquoise area), you can add this without making things seem “monochrome” (trust me, statistically, monochrome is a bad way to webdesign, either by going all black-and-white or all one colour).
I like the idea of colour schemes, but I thought I’d show everyone how one ought to use colour schemes in practice. So here you go.