detect spray color from an image

I got an image of yellow paper and some spray on it. I try to display only the blue spots in the image so that all the other colors will be displayed as white and everything that is blue will be displayed as black.(All the spray color will convert to black...) The problem is that there is a lot spots of spray in the corner of the picture which i want to display but when i try it, it always display also yellow parts which i doesnt want. I try to use so tresholding and imbinarize but they both didnt work well.

here is a the image of the spray and some results i achive. As you can see there are alot of duts that were converted to white instead of black and some yellow parts which converted to black instead of white.

If someone has any idea how i can detect all the spots of the spray without the yellow parts i will appreciate it.

[Rik]

Without your code it is difficult to know which threshold you need to adjust. Have a read here and here. It will greatly improve your chances of getting an answer.

Answer 1

[cr]

You should try colour based segmentation, roicolor(), etc.