First off, you're going to be hard pressed to find any anodized bars in general, since probably 99% of BMX bars are powder-coated chromoly, and most of them come in one color: black.

Secondly, what exactly are you asking? For a bar that will work with a stem with a 31.8mm reach? Because as long as your stem can use a bar with a 7/8" clamp tube (which all BMX bars I've seen, ever, use for the clamp tube), any bar will work. I think that the real question that you're asking is one of rise, upsweep, and backsweep.