The numbers on the tyres is the Shore rating, which is a guide to hardness. Generally speaking, the harder the tyre the less grip you get, but...
...then we have to consider the type of rubber. Yellow and Pink rears have roughly the same Shore number, but they are used in very different situations. Usually, where one works, the other won't. This guide on the CRC site gives you all the differences worth knowing, but take the usage guide with a pinch of salt.
http://www.teamcrc.com/crc/modules.p...article&sid=27
As a general rule, with today's cars, tracks and additive in the UK, we tend to use pink, yellow or magenta rears, and double-pink or magenta fronts. CRC, BSR, Mobgums (Jaco on a grey wheel), Parma (Jaco on a black wheel) or Jaco are the usual suspects and the most popular. Like Mark says, pick a make and stick to it.
I've used the Xceed tyres and found them good. They lose about -.3 seconds on a 12 second lap to the CRC or Jaco tyres, but that's a couple of missed apexes which most us could tighten up anyway on our current *cough* erratic driving styles. I have medium fronts only, and soft or medium rears. They grip well, wear well and don't go soft with the oily additives. Frankly, if you are starting out, then three sets of each rotated during the day will be an economic and competitive choice.
Find out what the regular drivers at your Club use for compounds and get those. Add other tyres as you need them. If you're at a National, come and ask people like Mark of me and we will gladly loan you tyres to try before you buy. True them down to 45mm rear, 43mm front, rotate them from side to side each run and run htem until they are too badly frayed at the edges to go on, or until they are 40mm rear and 39mm front.
The closer the stagger you have (rear diameter to front diameter) the more rotation the car will have in a corner. 2mm stagger (eg 45/43) is common, but on some cars and especially in Spec class, 1mm stagger is worth a try to get higher corner speeds. HTH
