Tone matching is a skill that isn't hard to work on.  The idea is to be able to properly hear a note and reproduce it.

Play a note (any note) on an instrument and have the student sing the note. Work on getting that sung note in tune...or at least being able to differentiate between it and other notes.
Ear Training
Some Good Ear Training Resources Listed Below
This is not the typical approach to ear training, but it is said to be very effective.
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Today in Music History
Turn it into a game. Listen to and reproduce two notes. Three notes. How far can he go before making a mistake? 
Not only is this a great memory game, it lets you hear how the student hears the notes.

Further advanced than this, ear training is a great thing for musicians.  The ability to properly hear individual notes, notes relative to keys, and intervals is especially important when you play an instrument where the notes are not individually delineated, like on a piano!

It's not exactly violin, but it is an important musical skill!
EarMaster is an Excellent Software package for Ear Training! Just Read the Reviews!
Violin Tip of the Day - April 5
* Violin Tips for Practice and Rehearsals.

* Violin Technique and Exercises.

* How to Care for Your Violin.

* Inspiration and Motivation for Violinists, Fiddlers, or
   Anyone Interested in Stringed Instruments.
Violin Tip of the Day -- Violin Student Central -- February 1
Today's Tip in a Nutshell:
Important Skill:
     Hearing, Matching, Remembering Tones
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