I notice that even in my orchestra, the upper string players rarely if ever vary the color of their pizzicato. The bassists are very active in their use of different right hand placements for different lengths and colors of notes. Try these at home.
1. For the softest notes with the softest attack, find the middle of the stopped string length with the fullest flesh on the string, and release the string slowly with a large muscle. Great for piano and pianissimo.
2. For medium dynamics, find a point one-third away from either end of the stopped string length. If a harder attack is desired use less flesh and smaller muscles for the release of the string.
3. Loud dynamics require moving to a point at least one-quarter to one-sixth of the stopped string length from either end of the string. We often play pizzicato closer to the left hand than to the bridge. This seems to minimize accidental snapping of the string against the fingerboard.
4. Short notes of any dynamic are more easily executed toward either end of the string. The area of rosin toward the end of the fingerboard often is good for very short and very soft pizzicato, if the right hand finger remains on top of the string, only allowing the rosin to grip the finger.
Students often have a very weak and percussive pizzicato. Experiment with using more flesh and more whole arm weight in your pizz. Be sensitive to the speed of the release. Good luck.