Extract one color from multi-color image

I have trained and classified an image into four classes represented by the four colors seen in the result image. I would like to seperate this image into the different colours (i.e. red, purple, yellow and green). Actually, the yellow colour is the one that I want to quantify. So if I could only extract the yellow color from the rest so that i can “measure” it, that would be ideal.

I have tried a lot of the commandd in Image>colour but I can only seem to split the image into three channels (red, green and blue) which is not what i want.

I hope that anyone could help me figuring this out.
Kind regards

You can threshold the image. Each class corresponds to a specific value.
Image > Adjust > Threshold…

If it is RGB then first change it to 8-bit
Image > Type > 8-bit

Thank you very much :slight_smile: