From a performance point of view, I would say stick with the same parchment pattern in different colors. I agree that your solution is cleaner - that's what I'd prefer - but you're only going to alienate users who possibly receive decreased performance on the client end (users with low-end tech? mobile devices?), whereas the extra bandwidth you would otherwise save using the monochrome pattern is (IMO) less likely to alienate users based on bandwidth (how big a difference are we talking?). I can't find any actual benchmarks for these situations.
Here are some links that discuss some related aspects of this. Note the first link has a user saying there are memory leaks in PNG alpha 24-bit in IE8.
Which is superior, CSS transparency or PNG Transparency?
CSS opacity performance. Image fading