An arbitrary mapping between colors and numbers (including one that's derived from mapping, or the periodic table, or whatever) is valid -- because the source of the mapping isn't important, nor does it need to be understood to guess the rule.
Ahh *lightbulb moment* Because the students could have guessed ...if and only if it contains a single stack and the number of pips in the stack is 3,4,5,6 where the top pyramid in the stack is red, blue, green, yellow respectively. and that would be deemed equivalent, as the master would not be able to disprove it, and it removes any language dependency. Thank you for the enlightenment. Timothy