As for pairing coloured garments with other coloured clothing, you can make an impact by contrasting your garment with a complementary colour. Use the colour wheel, find your garment's colour then choose another item of clothing in the shade that sits directly opposite. Green, for example, really pops against purple.
For a more muted look, keep things monotone. This doesn't mean black and white though. If you're wearing a bright pink t shirt, find the segment on the colour wheel and add items to your outfit that are either lighter or darker than bright pink e.g. light pink or a deep magenta.
Feeling more experimental, or want to learn more about colour theory? Resources like Adobe Kuler can teach you all you need to know - despite being created for designers, it works across the board for fashion, interiors and beyond.