Thresholding close colors

[nseiter]

Hello,

I am trying to use the "Threshold Colour" plugin to separate beetles that have fed on soybean foliage from beetles that have not (a dark green color from the beetle gut contents can be seen from the exterior of the beetle in its abdomen).  I have been able to do this with one exception:  I cannot make it separate these beetles from those that have fed on corn foliage, which has a distinctly lighter green color.  Are there any "tricks" I can use to separate color values that are quite close like these?

thanks,

Nick Seiter
Grad Student
Department of Entomology
Purdue University
[e-mail]