It occurred to me recently, after noting how grimy the matte white toptube on my Cervelo has gotten, to ask my buddy who is a professional car detailer about what to buy to maintain a matte paintjob. He immediately said McKee's 37 Matte Finish Cleaner and Protectant. And while it is practically chain lube expensive, holy crap does it work. If your matte finish looks pretty clean, it is not. I turned three brand new microfibers varying shades of gray and brown. Stuff really, really works. And as I'm only using it on a bike frame, that $20 16oz bottle should last for ages.

I now think this is why like 90% of matte bikes are black-- it mostly hides the grime.