Why are cities highlighted yellow on state maps

just wondering.

An area of a map that is highlighted yellow means there is another detailed map of that area somewhere else in the atlas, usually on the previous or next page. Since cities have many roads very close together, a very detailed map will be needed to find your way. This map is provided if the city is highlighted yellow.

Big D is correct, cities are often highlighted yellow in a collection of maps (such as an atlas) if there is a supplemental map for that location provided elsewhere. This is also done for inset maps (the little maps that show another area, sometimes in a larger scale, on the same page in its own little box, such as is done for Hawaii on maps of the USA).

Sometimes, though, places are simply highlighted to convey its individual importance compared with the other locations. For example, a capital may be highlighted in a state map, or at least its border may be, etc. This is not done as often though, as map makers generally place a star or use larger, bolder font to denote places of specific importance.