I have only just learned something.
"Curry" is not a particular spice. It is in fact a mixture of several spices used to make a curry (or sauce). According to the RecipeZaar, it can have 20 different spices! It is the tumeric which makes it so yellow. The powder we get in stores is like a standard mix of the spices from Britain. It was made so you could add the flavor of Indian spices without preparing it all yourself. In India, the spices vary depending on region and household.
Some recipes for "curry powder" are here:
Curry Powder Blend
So now I have learned something new today...and I have to see about making my own.