In my family, the answer to this one....
Why haven't they changed the colour of your money?

is because then we can't make fun of our Canadian family about their money. When the US started adding some color to our money, it destroyed YEARS of teasing within my family. Couple that with the toppling of the US dollar and now the two being equal in value, well I'm running out of things to tease my cousins about. Yet they have new things to tease me with - I guess turnabout is fair play, as the saying goes.