The seeming imbalance in the colour rarity and cost is very much an intentional part of citadels. It fits both the theme and the gameplay, making the merchant a very fast way to accumulate gold, which is what the merchant is meant to do.. Blue and red are identical as far as quantity (11 districts, 4 unique types) and cost go, while yellow has 12 but only 3 unique districts, combined with a high gold cost, in order to balance the power of gaining the crown. The fact that the non-green colours are fare less common prevents the merchant from losing his job as the main money maker, and balances out the power of the other abilities. The distribution of districts is not a design error, but a well tested gameplay tool. I now appreciate the consideration that was put into citadels, even on a level that is at first not noticeable. I do like the idea of adding your own districts, particularly purples, and i think that this could be a fun game mechanic experiment, if i could find a group willing to test it.