There is a new 'rule of thumb' out there- wearing 3 colours intead of two. I know it sounds simple but it makes a world of a differnce in your overall outfit. Wearing three colours instead of one or two just makes your outfit look that much more pulled together.
For example: If you're wearing grey pants and a black top- throw in a coloured belt or fun coloured shoes. When wearing neutral colours it is best to throw in bright colours. When wearing bright colours- throw in something neutral. Simple enough.