when I used pastels I was worried about darkening but I use ladies cheap hair spray as my Fixer and I have found one this is cheaper then normal fixer and light spraying on your work will darken a bit but I found it brought the colours out hope that may have helped you
Hi Christopher - I use Krylon Workable Fixative. It is also a UV protectant, something I don't think you will find in a hairspray. I use it on pastels and colored pencil pieces. The darkening of the lighter colors is very minimal if there is any at all. I use very light coats and have never had any issues with it at all.
Good luck! and Happy Arting!!