Rainbow (color) charts are control charts that allow project management teams to see values such as data, trends, and runs on a single chart. To distinguish one value from another, each has its own color. For example, a yellow bar could signify the values between one and three sigmas. Rainbow charts can be used for one specific value, or they can be applied to all values being currently used.
Rainbow charts basically look like horizontal bar graphs with shaded areas. These areas will represent certain control values, and more than likely they will be different sizes depending on the sigma area. For example, generally, the yellow and green zones will more than likely be the same size since they represent a total of two sigmas. The red zone will be the smallest since it represents anything outside of three.
Control Limits Chart
Rainbow charts are broken up into two categories: control limits and specification limits. The control limit rainbow chart usually has three color zones: green zone, red zone, and yellow zone. The green zone is generally the plus or minus one sigma area, and these values are deducted from the average X denomination.
The yellow area of the chart is generally designated for the one to three sigma area, and the red zone is anything outside of the control limit. The red zone limits are defined as the Upper Control Limit and the Lower Control Limit. Basically, the Upper Control Limits are values that fall three sigmas above the average X value, and the Lower Control Limits are the data points that fall three sigmas below the X value.
Specification Limits Chart
The specification limit control chart is also broken down into red, yellow, and green. In this case, however, the red zone is anything that falls outside the specification limits. The yellow zones are those values that fall between the control and specification limits while the green zones are values that fall within those specifications.
Rainbow Chart Use
Rainbow charts allow teams to quickly see which values are within standard procedures and which processes are outside or at unacceptable levels. If most values fall within the standard (green) zone, then the procedure is generally within control. And, the closer that these values are together, the more accurate the processes are.
If, however, most processes are within the yellow or red zone, then the procedure may be out of control. Standards are no longer being followed, and action must be taken.
Having these charts provides teams with early warning signs that their procedures are not working. While work can still proceed as normal if the values are in the yellow zone, this will let project managers know that one of the tasks or procedures needs to be reanalyzed or changed.
Production, however, should stop completely if more than one or two values are in the red zone. This will indicate the entire process is out of the control.