Keeping a swatch book of colors that you like and constantly use is always a good idea. I actually realize that it is very helpful especially if you are like me - like mixing your own colors. Here's a photo of the grays that I am working on. I wanted to make sure that I have at least two grays in my palette - cool and warm. I was able to find a cool gray in the store (I liked the Amsterdam Acrylic Neutral Gray color), but the warm gray - not really. Considering that I actually wanted a "fancy warm gray" - a lavender gray. See below here, on the photo you can see my progress of achieving this very nice lavender gray color.
Well, whatever it is, i know that next time, when I run out I will be able to re-create this beautiful color, because I keep a record of it in my swatch book. Cool hey!