There is a recent documentary on Perlman that is worth watching for one reason in particular: the discussion of color and vibrato. Perlman describes vibrato not as an ornament or a habit but as a palette — different speeds and widths produce different colors, and a violinist who only has one vibrato is like a painter with one brush. A narrow, slow vibrato pulls the sound inward and dark. A wide, fast one opens it up. The choice is not decorative; it is the interpretation.
What makes Perlman’s authority on this subject unusual is that you can immediately verify everything he says by listening to him play. The vibrato changes constantly — consciously, deliberately — and once you hear it that way you cannot unhear it.