This can be a very big subject, but keeping it to a few key points here are a few guidelines.
When it comes to colour it is good to find combinations that work well or complement each other rather than clashing. Some say blue and green should not be worn together or pink and orange. But blue or teal and orange or green and red go well.
The way you can tell is to look at a colour wheel. Colours that complement each other are opposite each other and are either across the wheel from each other or adjacent to each other. Hence the teal and orange. But colours that are adjacent but skip a colour tend to fight each other.