I know it's kind of thread necromancy, but i think i can contribute something (also, first post ever, Yay!): I came back into the hobby a few weeks ago after a hiatus of a couple of years and started with a bunch of inquisition stormtroopers/kasrkin which i hope to be posting pictures of in the next few days. After having finished the first five guys and now working on a few additional henchmen/advisors and a AM comissar, i would like to share my experiences with (almost) completely mixing my paints myself.
I have actually used the Army Painter Warpaints for this instead of citadel paints, and apart from one or two exceptions i was able to adapt the recipes in the first post. I just replaced the citadel white, black, red, yellow and blue color with their (more or less) equivalents in the Army Painter Range, namely
Citadel Paint-> Army Painter Warpaint
Chaos Black -> Matt Black
Skull White -> Matt White (OK those two are pretty obvious)
Blood Red -> Pure Red
Enchanted Blue -> Crystal Blue
Sunburst Yellow -> Daemonic Yellow
So far i've had pretty good results with the following recipes: Red Gore, Bleached Bone, Dwarf Flesh, Codex Grey and Goblin Green, as well as a few variations and mixes between those and the "base" paints. The mixed paints turned out pretty much the same tones as the corresponding citadel paints, at least as far as i remember (the last time i was active, the citadel paints actually were still called Blood Red, Red Gore, Bleached Bone and so on... to give you a small estimate of the time that has passed). What i have been struggling with are the darker greens, namely Snot Green and Dark Angels Green. Those always turned out with a little to much of a blue hue to them, but adding more yellow made the paint too bright too fast. I might experiment a little more with that and let you know of the results.
All in all i think this is a worthwhile way to create your paints, as long as you are okay with the extra time the mixing takes. That extra time of course gets greatly reduced once you've passed the "experimental" phase and begin "mass producing" (as in mixing whole pots) your new paints. Dropper bottles definitly help greatly in achieving consistent mixtures. I highly recommend using them! I guess in the long run you will be able to save a bit of money on paints (especially when buying non-citadel paints for their higher money-to-actual-paint-ratio), which is a big plus for me at the moment, being a student and having rather limited hobby funds. Also i can see uses for trying out new colour schemes, so you don't have to buy every paint only to find out that your scheme doesn't work. Sadly, all this only works for non-metallics (at least i never was able to mix metallics with anything and not get really horrible results)...
Wait, that might actually the incentive for me to start learning how to do NMM... I'll keep you posted