I just looked at the CDNN website and found they have grey colored (supposedly saltwater-resistant) slides for .40/.357 SIG P229's.  They are used but in very good to almost new conditon.  They are complete slides--no barrel or recoil rod/springs.  They are $129.99 plus shipping.  I have always liked the looks of two-tone SIG's and am thinking about it pretty seriously.  I would have to have my night sights put on the new slide if I wanted it to be my primary slide.  Maybe I could buy a .357 barrel and new recoil rod/spring and be able to change the whole slide assembly to change calibers.  The P226 (9mm) slides are only $79.99.  I'm not sure why there is such a price difference.  Is $129.99 too much?

P229 slides are already stainless.  Spend your money on something more useful.  

If you're only in it for the two tone look, send your slide to Tripp to get hard chromed, it'd be cheaper and ever more durable than 229 slides are already

And to switch calibers, all you need to do is swap barrels.  If you WANT a new slide, go for it, but you certainly don't need it.  The P229's slide is stainless, and if its black that means stainless Nitron, about as durable as it gets.  +1 on hard chrome is you want twotone.