Buying tip of the day; expansion tanks that are dulling and appear yellowish are very close to being either cracked and not leaking yet, or leaking and hiding the result in the trunk.
Of course you can see the difference between an older tank and a newer. The top tank looks yellowish and worn out, the lower tank looks clear and intact. I've purchased 2 Boxsters with yellowish tanks, both needed replacement within 1000 miles of ownership.