Usually the rule of thumb is to stick to the colour that's in there... but if it's brown that's no help. There are 3 colours... blue, green and pink. I would go with what the manual says as it will suggest the right one to ensure the right amount of corrosion protection and lubrication (not sure if you a manual to hand?).
Going back to the colour comment, this isn't always the best rule of thumb as someone may have changed coolant colour so it may be the wrong one in there. Would probably suggest (and maybe you were planning this already) to do a coolant flush. For the 1st 2nd and 3rd gens they recommend glysantin based coolants. Here's the stuff i use:
http://www.amazon.co.uk/dp/B003BPNOXQ/r ... 2JTDPMWD38
Use the zoom picture feature and look at the BS requirement number and as long as it meets or exceeds the number recommended, then it will work.
Confusingly that coolant i mention is a blue-ish green colour. I read a whole article about coolant colours just dyes and the contents don't actually change. Not sure if that's true or not. Good times!
Hope that helped more than it caused any confusion
