The spells in each colour are reflective of common associations with that colour, including personality traits and temperament. As we are very different people, our natural affinities are also different.

[There are hints toward what colour would favour Mello in all of that. Indigo is a very cerebral style of magic, mostly unemotional. With their upbringing, most children from the House would have some chance of learning it, even if it isn't the best choice for all of them. Mello will be capable, he's certain, but he will not be a natural Indigo; he would be a natural Red.]