Having trouble figuring out value relationships in the field? A red filter lets you see the landscape in shades of red. It makes it easier for the painter to see what's light and what's dark by removing the issue of color. You end up comparing light and dark values of reds only, which is much easier than comparing a full gamut of colors. A red filter (shown here), or my personal favorite, a fashionable pair of red secret decoder glasses, is just the ticket.
See how the world looks through the filter?
|Through a red filter|
But there's a danger in using a red filter. Red in a scene will appear much lighter, leading you to make mistakes in your value analysis. Look what happens when you look at red through the filter:
What's worse, red also makes green and blue look darker! So, use the red filter with a degree of caution. At best, I consider it a "training wheel" for beginning plein air painters. Use it if you have to, but then train yourself to analyze values with the unaided eye.
You can find more helpful tips and tools in my book, Backpacker Painting: Outdoors with Oil & Pastel, available at Amazon from this link.