Why do Butterflies have such colourful wings? And why do you never see a really boring butterfly?

The Cabbage White isn't particularly colourful or exciting:

http://insects.tamu.edu/images/animalia/arthropoda/insecta/lepidoptera/pieridae/pieris_rapae_adult_dorsal_m_18.jpg

But generally the colours can be for any number of reasons; mate attraction, camouflage, as a warning to predators (i.e. indicating they are poisonous [or pretending to be poisonous]).

"Boring" is a highly subjective term! What seems boring to you may be the latest in mating rituals to another butterfly. You should also take into account that many insects are capable of detecting wavelengths outside of our visual range, so where you see a single colour, to the eye of a butterfly it may be lighting up like a christmas tree. Have look on google and you will find some striking examples of this kind of thing in flowers.