When I ran the macro in my version of ImageJ it worked fine. Turns out the "RGB to Lab" function was part of the older versions of ImageJ and is no longer included. Go to https://imagej.net/Fiji/Downloads and download the 2014 June version. The paper you are referencing was published in 2011 so they were using the older version as well. Also do not update ImageJ as this will remove the "RGB to Lab" command.

Thanks, Erik!

I had tried installing a Threshold_colour plugin by G. Landini, as it's named in the paper but with a broken link (Actual link: http://www.mecourse.com/landinig/software/software.html ). It solved the "RGBtoLAB" error, but the macro didn't work since it didn't select the areas at all.

I migth try with ImageJ/FIJI old version as you say.

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EDIT: I tried with FIJI old version, and the macro worked. There are some things to adjust, if possible, but it works quite well.

Thanks.

Another approach available:

please read https://blog.bham.ac.uk/intellimic/g-landini-software/colour-deconvolution/

together with settings for PicroSirius Red from http://dev.mri.cnrs.fr/projects/imagej-macros/wiki/Fibrosis_Tool

Works perfectly in rat hearts after LAD occlusion

Hope it will help!