The red/blue test

When working in color, Maurice [Noble] would check his values using red and blue filters, sometimes called monochrome filters. Generally he used red filters, but if a composition had a lot of red in it, he used blue. Using filters reduces color to simple tones, allowing an artist to see and compare values in a composition. This was important to Maurice in making sure the highest area of contrast was either the characters, or the area around the characters.



The Noble Approach: Maurice Noble and the Zen of Animation Design - (link)