Color standards for control charts

Question

Are there any references or standards that dictate the color coding of the zones of a Shewhart style control chart. We use green in +/- 2 sigma range, yellow between 2 and 3 sigma and red beyond 3 sigma. I am hearing that using green, yellow and red is not good because of other connections (like a traffic light) we have to those colors in society. Will you please reference your response? Thanks

Answer 1

I don't know if there are standards, but the red/yellow/green sounds like a good first try ("warning"/"caution"/"okay") precisely because of traffic lights. As long as that's what you are actually trying to convey. However, the red-green colorblindness issue is a problem.

If you're using R, there is a package called qcc which does Shewhart graphing and I assume they've done some amount of research as to what is the norm. (They appear to use red points to highlight potential outliers.)

You could use colors and shapes: maybe circles for green, triangles for yellow, x's for red, or maybe dots for green, circles for yellow, and circles with x's for red. You could do this with color and shape, or just shape.

You could also use background shading: for example, shade the background of the graph so that it is a solid mid-gray inside of +/- 2 sigma, then a light gray from 2-3 sigma, and white beyond that. Plot black marks on this and contrast will make your point: black on gray stands out less than black on white, so normal data stands out less.