> Am I allowed to mention the prime number or am I not? Since "the colour" is the whole context which led to you mentioning the prime number, the answer is "it

> Am I allowed to mention the prime number or am I not? Since "the colour" is the whole context which led to you mentioning the prime number, the answer is "it depends". In practice you can publish it as a part of mathematical paper on primes. You cannot publish it saying "here's a prime that decoded as X does illegal thing Y". Anything in-between is gray zone, with the disclaimer that courts don't like "clever" schemes that try mess with the "colour". So if you try to publish a fake mathematical paper to thumb your nose at the illegality of publishing something, the court will be unsympathetic.