I worked out how to get the image_blend value for one of my custom colors by using the debug log, but I am sure there must be a better way.
Update: Just discovered that with the example of the spring which has an image_blend value of 3452415, that this value is obtained by converting the Hex value (FFAD34) to a decimal value, but only if the colors are reversed (ie entered as 34ADFF in the calculator).
I still wonder why the approach of using image_blend values in the switch statement is used.