This way of solving the problem is brilliant in my opinion. A fact though is that the taks description said "recursive". So it's not a solution to this problem.

it doesn't specify in the description that empty list should return nils. User has to figure it out when trying to submit the solution

I think it's not described clearly or description is overcomplicated, especially with the negative parts, especially regarding that t's just in definition for odd number

tests were not very clearly defind. you have a function yet it's supposed to respond to the_function[element_num] which is an array behavior

I don't think this Kata is correct as well. I used more different solution (with *args parameteres). Tested my solution in the console (ruby 2.0.0) and it works perfectly. When trying to submit, it throws strange errors.

please, correct the function name for the Ruby kata. It should be snake_cased ( like number_to_string) istead of CamelCased.

I agree with eik3 definition of the expected task. I also though it should assing color only upon initialization. Yet Denym is totally right. If so, then the kata creator should of defined this with a test.

I did the same solution, but in addition I've added a regex to evaluate if we are having up to 26 lowercase chars. I think arg validation is important.

That's not clever. Target was to output is as over-complicated as possible. Not as simple as possible.

Isn't better to use oneliner like:
def square a; a*a end