Principles of Design 4 of 7
Or as I prefer to call it 'Consistency'. It's very easy to feel like a kid in a sweet shop when you browse through all the fonts at your disposal, or open up the colour picker and start playing with the sliders but throwing everything and the kitchen sink at your design is one of the worst things you can do.
Chose no more than three fonts and use them across your marketing and design. Stick to the colours of your brand with maybe one additional neutral or accent colour (depending on you brand colour pallet). Remember you want your customers to instantly identify you when they see your marketing not get confused.