If you use standard white-spirit-like as a fixer, once it's fully dry, the pigments get back to their original color. Maybe a fixer based on a very diluted varnish or glue can modify the color, especially if it's not perfectly flat.
However, spraying a mist of thinned matt varnish over the pigments is OK, but NOT a wet coat !
The best way to experiment is... to experiment ! Just take a plastic something, paint it the color you want, and play with the pigments, try various things, so you will see what you can achieve, or not, without taking any risk with your model.
Following the big Photobucket rip-off attempt, all my pictures on the forum cannot display anymore. I try to reupload the RFI pictures, but i'm not sure i'd have enough time and energy to reupload the pictures of the WIP threads. If you're interested in a specific subject and want me to reupload some pictures, just let me know.
Finished : F-16C Barak, F-4J VMF333, Czech Mig-19PM, Bell X-1, Russian Su-25 in Chechnya, Ta152H-1, Czech Avia S.199, Heinkel He162, Swiss AF Bf109G-6, Regia Aeronautica Bf109G-6, P-47D "Slick Chick", RFC Sopwith Pup, A5M2 Claude, Schneider Cup 1926 Macchi M.39, German F-104G JaboG34, Curtiss P-6E, Curtiss P-36A USAAC, Piper SuperCub floatplane (PA-18-150), Russian built Nieuport 11, Albatros D.Va Seefrontstaffel 1, Bf109G-12 (two-seater trainer), Texas ANG F-86D, Post-war Hawker Tempest Mk.V