Lipstick - what's that you're supposed to do when you apply a lighter colour?

[ScareyCaligulaCorday]

You know you're supposed to put a lighter colour in the middle of your lips, or a darker colour maybe? Anyone know? And does anyone know why?

lighter colour in the middle is meant to make lips look plumper

can look very stupid if not carfeul - better to lipstick all over then gloss just the middle

Thanks MrsB, will go and try it out now