Filament varies, and between the dye content and impact additives, I find variances between different colors from the same manufacture. I find the easiest way is to pick an object you will use to calibrate consistently over time - I use Moria's Treefrog - and then dynamically adjust your e-steps to get the proper extrusion.
There a great example / description of doing this that Triffid Hunter wrote a while back -
http://reprap.org/wiki/Triffid_Hunter%2 ... tion_Guide - that describes a method of watching the infill to see when it's "right" to fine tune your extrusion. I find this helps a lot, and gets easier as you do it more.
Also with PLA - most of it will extrude over a *large* temperature range, and in general has a more matte finish at the lower temperatures and more glossy at the higher. I've got some Protoparadigm that will print happily from 165 to 215c. Since heat is the devil for PLA, you really want to go as low as you can - unless you are shooting for the glossy look - and then cool the heck out out it.
Here's something I just knocked out that I think came out stunningly well, in Diamond Age Black & White.
- Yin & Yang Vase, 195mm http://www.thingiverse.com/thing:85951