I have a $3 "Gray Scale and Value Finder" that I have found useful in my studio. It is available from
http://www.colorwheelco.com/
Here's how I use it: When I need to match a color, I mix a pile of neutral color to get a small quantity of paint that matches the value of the paint I am trying to match. (This "neutral" is usually raw umber + white.)
To this neutral pile of value-matched paint on my palette, I add pure color until I get the match I'm looking for...