In my opinion if you don't visit a professional for a colour analysis then you're relying on luck to get it right.
If you're not sure if it's something you really want to spend your hard earned money on then think of it like this: over the course of your life how much money have you spent on items of clothing that have sat unworn in your wardrobe for months or even years? Chances are you ended up giving them away to a friend or charity, or selling them at a loss on eBay. A colour analysis will save you money in the long run since you don't make as many bad buys.
There's a discussion at Mumsnet comparing the two companies to help you decide who you'd prefer.
I can't give an un-biased opinion personally as I haven't tried both companies but if you live in West or South Yorkshire I would certainly recommend Karina Leacock from House Of Colour. You can find Karina on Facebook.