Value is of paramount importance while painting, and this is why there is no more a sinking feeling than when you lay a stroke of paint down on a wet canvas, just to find that your dirty brush has shifted the tint or tone of the area you were painting. Do you scrape, do you paint the correct value in thicker, do you add a darker/lighter value and hope you mix it on the canvas back to where you intended... Even painters of "mud" don't want to accidentally shift their values. To avoid this, many artists clean their brushes often, or use many, many brushes in each session.
The paintbrush holder pictured here is a great tool to help you avoid the problem of crossing values. It consists of 33 slots organized into three tiers of 11 holes per level. The diameter of the holes reduce in size as you step down, enabling small brushes to fit snugly in the slots on the bottom row, medium brushes in the middle, and large brushes on the top row. From left to right in each row, the holes correspond to values as established by Albert Munsell's color system; from 10 (white) to 0 (black), with nine equal value steps in between. In each column, the brushes are dedicated to a specific value, regardless of the hue. This way all you need do is wipe the excess paint off the brush as you switch to a new hue of the same value. The benefit of this is that it prevents value shifts created by the deposit of unintended leftover paint which had remained in the bristles (providing you put your brush back in the right spot), and it eliminates the need to clean the brush with solvent between switching colors (and taking the risk that leftover solvent will be transferred from your brush to the painting, damaging a finished section). It also keeps your brushes from rolling onto the studio floor!
I based my brush holder off of the one Marvin Mattelson used when I first studied with him. Knowing how he lays out his palette, it makes perfect sense to use a holder like this with a value based painting system. Marvin later made improvements to his version, including stepped slots so the top row can hold all three sizes of brush handle (I tried this using 3 different size drill bits in each hole, but it didn't seem to work as well as his special bit, which was designed to cut all three widths with a single pass), and angled slots (90˚ in the top row, 60˚ in the middle, and 30˚ on the bottom) so the brushes were fanned out, making it easier to take and replace brushes (in mine, the brushes are all aligned perpendicularly). Occasionally, he makes a couple of dozen of these and sells them to his students, but only when his schedule allows.
Someday, when I can set up a woodworking shop again, I hope to make more of my own studio furniture, and more painting tools like this one.