I just thought about it and I didn't find any way to make it clearer since as you've already noticed, instructions from filejoker themselves are unclear.
Also, at least with the way it is currently made, there is not much more space in that captcha or we'd have to include the instructions at the bottom of the image to make them more precise.
E.g.:
Code:
Find all image numbers of similar looking shapes regardless of color and enter them comma separated.
The number of results may vary but usually there are at least two similar shapes to be found.