I use these Gamblin colors to grey down passages - although for a subtle and luscious grey, nothing beats Cobalt Violet. I ran out of the tube blue in the middle of one night and wanted to keep painting. The thought crossed my mind I could make the color if I maintained the authenticity using my favorite Gamblin base colors.
On the back of an artist's color in any medium, is a series of letters paired with numbers to identify what's in the tube. I researched the Color of Art Pigment Data Base, went to number PB 29, and found Ultramarine Blue. PW 6 is basic Titanium White - the two ingredient colors of the blue. The mixing process can become a lot like eating spaghetti and ending up with the right amount of sauce. Approach the mixing lightly until you're on track, then make enough to last for the painting session. I keep small glass jars around for the times when the concoction multiplies beyond the original intention.
Occasionally, I may want a favorite straight from the tube for convenience, however, in a pinch it helps to know how to make the color you enjoy using. Plus, there's more creative and financial freedom when I purchase basic colors and use them in multiple variations. Golden Artist Colors publishes a great chart of 46 extended palette colors from 7 basic pigments and white.
The results of the experiments are left to cure on Cottonwood Arts Rounded Canvas Sheets. They come in various sizes, the 5x7 is my fav for color notes, quick plein air sketches or sending original cards - and they're easy to file.