I think the only way to tell what your "undertones" are is by comparing with other people. It scared the life out of me when I realised how pink I was comared to how yellow others were, but as I fry in the sun, I can't think why it came as such a surprise. According to the time of year I use BE in Fairly Light, Light or Medium Beige, sometimes mixing them. I LOVE that I get full coverage of my dreadful skin, but nobody knows I'm wearing make up, a HUGE bonus after years of wearing mask-like foundations.