A while back I became interested in learning about traditional paint colors, particularly those commonly used for portraits and landscapes. A trip to an art store leads to a plethora of colors, most of which are mere convenience colors (they can be mixed from other colors). So after some research I made my own list of colors to narrow down my palette. The image below shows how I went about this - I made a chart with the most commonly used colors, from the paint lines which appealed to me most.
It was surprising how widely the colors varied between manufacturers. Also, some manufacturers didn't produce the exact color I was looking for so I chose the closest match, designating them with a dot. Instead of relying on any one manufacturer, I ended up mixing my own version of each color to suit my particular tastes (as seen in the 'Mine1*' column), and testing them on the little face sketch on the right. Of all the manufactured paint lines, I preferred the Old Holland oils most. I also ended up discarding several of the colors in my final set, keeping only those checked on the left. I keep almost the full set of these colors in the Old Holland line on hand, however, just in case I need them.
In practice, I don't actually use these colors much, but familiarizing myself with them benefited me greatly. If you're interested in learning more about traditional color palettes and how they can be used, I highly recommend James Gurney's blog, as well as his book Color and Light: A Guide for the Realist Painter, which gathers many of his blog posts into a convenient reference guide.
If you want to download these swatch sets to make your own, I've created another blog post which includes the full list of colors in most of the paint lines shown here. Each set can be downloaded in Photoshop's swatch format (*.aco), which is also compatible with Manga Studio. You can find that post here. For other paint manufacturers, I've found that Cheap Joe's online art store has good color swatches from which to make your own sets (just look up a particular manufacturer and then take screenshots to sample from in the graphics program of your choice).