Since I'm a novice painter, I tend to go with "What looks close enough to what I want to accomplish?" I'm only painting to a tabletop standard and painting is supposed to be
fun (or, at least, more relaxing than my day job), so why stress about it?
I prefer Reaper paints for mostly irrational reasons. One benefit of this though is I can use Reaper's
Power Palette tool to find approximate paints to use. It is not necessarily going to be 100% accurate but it's a starting point.
Another thing I do is compare colors on painted miniatures at CamoSpecs against the swatches in Vallejo's catalog and Reaper's master swatch PDF to find an approximation. I assume the color balances are all more-or-less the same and any wackiness in my monitor will apply equally to them. (I also assume I calibrated my monitor correctly.)
I also try to look at the bottle of paint in the store to make sure this is really the color I'm looking for. If I'm shopping with my wife, I'll ask for her opinion as well.
Sometimes this still doesn't work out quite right. (I don't think Reaper's True Blue is a good alternative for Steiner blue. I think I'll have to get VMC Andrea Blue for that after all.)
If you're looking for paint equivalences and have an Android device, take a look at the Paintrack app. You can use its color tools functionality to find equivalences for a particular paint across several paint lines. I haven't compared it against online paint equivalency guides but I assume it's comparable or better. I still wouldn't make a decision based solely on it though.