|MadSci Network: Zoology|
This is completely guess work on my part.
I expect that tortoises are of sufficient level of intellegence that they can learn. Perhaps they learn that different foods are better or that other animals are dangerous. Colour is on just one part of how they would interpret their world. Shape, texture, smell, feel, shades may all be part of how they understand and relate to things around them.
There may be ingrained or inherited instincts related to colour.
You would need to study the tortoises in their habitat over a long period to understand how colour vision might be important.
Consider also that if they went to their familiar food dish that they may have found it by means other than colour. Of course I don't know how you set up your experiment.
Good luck with your studies.
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