You already prepared a nice image. From the RGB, convert the image to grey. As you have only a few well-defined colours, they map to four distinct grey values. These can be measured individually by setting a threshold (Image>Adjust>Threshold) and measure, while ‘limit to threshold’ is set in Analyse>Set Measurements. While doing so, I Plugins>Macro>Record 'ed this macro:
run("Set Measurements...", "area min limit redirect=None decimal=5");
Yellow is converted to grey which falls in the range setThreshold(159,171); is this what you need?