The instruction for this is from one of my Virtual Art Academy lessons on plein air equipment.
I tossed my old white glass palette in favor of a grey one, added a grey color isolator to my equipment, and attached a value scale to the glass and the panel holder of my box. I'll show you how I did it.
For the Value Scale:
I used a Glidden grey paint chip from Hardware Hank and cut it in half lengthwise so I had two narrow strips of value scale. I used clear shipping tape to attach each half to their respective places. I was careful to make sure the one on the glass was in the same position -light to dark - as the one on the box.
|My value scale from a paint chip and my new grey glass palette|
For the Palette:
I got a piece of single strength glass cut to fit my box. I used a medium grey spray paint to coat the glass, after I had taped the value scale to it. When the glass was dry I dropped it into my box and it was ready to go.
For the Color Isolator:
I used the same spray paint to coat both sides of an old credit card. (A friend suggested coating the card with gesso first to really cover the numbers. I didn't do this, but you might want to try it. I'm not sure if the gesso would stick). I used a paper punch, which took some time, to put three holes in the card, evenly spaced. This is for isolating color and judging value when out in the field. It is my favorite new tool!
I've found this going grey to be one of the most helpful and simple changes I've made to my painting routine. It's been much easier to judge the values and colors as I'm working. I haven't spent much time actually consciously comparing things, but because my panels are coated a medium value (from the leftover paint scrapings) and my palette is the same value as the color isolator I don't have to mentally try to make adjustments for the different values, as I did in the past. And when I do want to make a conscious comparison I whip out my color isolator and line up my holes to see the actual local color and the color I have in my painting.
|A comparison made easy. I usually don't hold the isolator right on the painting, but this worked for a photo.|