Yup--adsorbtion versus filtration. Also, is the colored water a solution or a suspension?
Possibly a decent test for organics, probably invalid for particulates.
Depending on your food coloring, I strongly suspect that you aren't measuring the filter pores. food coloring is just a largish organic molecule - A chemical - which filters alone won't remove - so I'm surprised any filter would remove it - unless it has a charcoal or other active prefilter that gloms onto the organic stuff in the food dye. - but then you're measuring the fact that you have a prefilter that reacts with the food coloring, as opposed to filtering it.