We offer samples for some of our products for £1.00/£1.50 plus shipping so that you can check you've got the right shade before you invest in the full size - We'd recommend ordering perhaps 2 or 3. The samples contain around 0.75g of foundation powder which although sounds very small will be enough for several applications. You can even mix foundations together to create your own personal colour.
Have a good look at your skin in daylight. If you look like you have pinky tones to the skin, then your natural colouring is cool. If you look more yellow, your undertones are more likely to be warm. If neither is obvious, then the neutral toned colours will suit most skin types.
Try looking at the inside of your wrists too - if the veins show mainly blue, then you're more likely to have cool colouring, and if they look green then warm tones might suit you better. Again, if you're not obviously either one, then try the neutral tones.
It is also possible to tone down your natural undertones by using foundation a shade away from your colouring, for example, to tone down pinkness, choose a neutral or warm shade.