JQVMap's documentation says you should pass an object to the colors
paramerter...
Colors of individual map regions. Keys of the colors objects are country codes according to ISO 3166-1 alpha-2 standard. Keys of colors must be in lower case.
Here is an example of how it can be done...
var highlighted_states = {
il: '#f60',
mo: '#f60',
tx: '#f60',
ut: '#f60',
}
jQuery('#vmap').vectorMap({
map: 'usa_en',
color: '#ffffff',
colors: highlighted_states
});