In addition to discoloration, you often see the spacebar is a bit shiny too. This is evidence of excessive wear as the original textured surface has worn off due to use (slamming your thumb down on it thousands of times, combined with the effects of finger oil).

And so I honestly must say that this Mac 128/512k that I bought off EBAY was hardly used by its original owner. Because if he had used it often, the space bar would be shiny due to wear.