Why are asia and south america colored in a neutral color ?

Well it all depends on what type of map your are referring to Asia and South America may or may not be colored in neutral. My map however shows them in full color. There are alot  of maps created for different purposes so there is no way to know unless I know what map you are looking at. Usually, if they are grayed out, it means the creator of the map does not want you to focus on that area.

Well, a and b together would be negative. b alone would also be negative.