The titles of map view note icons are rendered in $NameColor, whose default value is "automatic". This means that in most cases the note's title is rendered in black (#000) or white (#fff) according to which offers the best contrast. In some cases, where neither black nor white may offer good contrast, titles use a yellow colour.
These automatic colours cannot be redefined, though $NameColor can be set to user-defined colour though this does mean the title colour will not change if $Color changes in a manner lessening contrast.
Automatic colouring was defined before more recent inclusion of new colour schemes, i.e. the 3 default 'automatic' choices were not optimised for all colour schemes.