I have always stayed away from fixatives for both reasons of toxicity and adverse influences to the appearance of the media itself. Understand that a fixative is a consolidant or varnish that physically binds a powdery material, such as charcoal or pastel, to the substrate. A common fixative for charcoal drawings has been a weak alcohol solution of a resin (shellac, mastic, or sandarac) applied with a sprayer or atomizer. However, these resins tend to discolor with age, so current commercially available fixatives contain dilute solutions of synthetic resins such as acrylics, cellulose nitrate, and cellulose acetate. Aqueous solutions of starch, gum arabic, funori, and casein have also been used as fixatives. While fixatives may protect and hold the pigments in place, they can also cause numerous problems over time such as changing the refractive index and gloss, darkening, cracking, and becoming insoluble.
There are some very informative posts sharing personal experimentation with fixatives. Here is one such post:
Additionally, Artist’s Magazine had an isightful “Ask the Expert” entry on this with Mark Gottsegen (founder and director of AMIEN (amien.org), a highly valued online technical service for artists, a long time professor of art and materials education at the University of North Carolina – Greensboro, and the author of The Painter’s Handbook),
This is especially pertinent regarding how some artists observe/describe lighter colors “disappearing” with the application of fixatives:
"Fixatives can alter the refractive index of many pastel colors, especially those light tints that contain chalk: The chalk goes transparent if the fixative is too heavily applied, and the colors can darken and change. But this effect can be mitigated by more carefully considering what a fixative is supposed to do.
A fixative must not be applied as a coating; it should be thought of as a “barely there” mist. It’s not intended to fix the particles of color to each other and to the support, but rather to gently prevent the dislodging of the topmost layer of pastel from the rest of the painting. Properly fixed pastels will retain their vibrancy of color, but they will remain very fragile. This is the reason many museums refuse to lend their more valuable pastel works to other museum exhibitions. I remember seeing an Edouard Manet pastel at the old Jeu de Paume museum in Paris that had a very fine line of color on the lower edge of the mat where the pastel particles had been dislodged by the vibration of footsteps from hordes of passing tourists.
To attempt to seal the pigment, it’s a good idea to practice your spraying technique. If you’re working on a pastel and intend to add another layer, use a light spray on the painting-in-progress while it’s still on the easel. When you’re ready to apply the final layer of fixative, lay the picture flat on a tabletop and, holding the sprayer at about a 30-degree angle at eight inches away from the picture, rapidly spray across the entire picture plane so the fixative falls in a light mist over the whole painting.
You can use any brand of fixative that’s labeled “archival” (though I don’t like to see that word used in this context). You’ll probably find that the best fixatives contain an acrylic or polyvinyl acetate resin, combined with ultraviolet light absorbers and inhibitors, a solvent or two, and, of course, a propellant.
Before you do this, let me offer a word of warning: Spray fixatives contain hazardous ingredients and can be dangerous. In fact, spraying anything can be hazardous. Read the label and follow all precautions, which probably include the phrase “use with adequate ventilation.” I’d rather wear a respirator or use direct local exhaust. At the very least, work near a powerful window fan." -M.Gottsegen
Hope this helps!