The Colored Noteheads on Strings plugin assigns colors to notes based on the string on which the notes appear. It uses colored notehead styles rather than traditional Sibelius coloring, which enables it to color individual notes in a chord.
Unfortunately it is not simple to determine which string a note should be assigned to, and this can require some setup before the plugin can be successfully used. The document "String numbers for notes in stringed instruments in Sibelius" included in the plugin zip file, explains how this can be done.
If you are going to do serious work coloring notes based on a string, you will probably need to bite the bullet and create a suitable TAB instrument, copy notes from the notation instrument to the TAB instrument, adjust any string mappings, and then copy the notes back to the notation staff.
For some instruments (violin, viola, cell, contrabass, guitar, mandolin, banjo), the plugin can guess which string to use, effectively treating the music as if it were all in first position. This may be good enough for simple music.
The document explains all this ins some detail.
Bob
An experienced user of Sibelius. Sib 1.2 - 7.5, Windows 7 Pro SP 1 64 bit, 8 G RAM. Year 2015.