Displaying Minifigs and avoiding discolouration
Taking a recommendation from our friends over on Photography there are a couple of options available to you:

The best and most common form of protection from fading would be the selection of UV glass or acrylic. Any framing store will have these options available at multiple price points with trade offs such as glare or less glare, scratch resistant or scratch prone, and tint(green) or no tint.
[Another option would be sprays - for example Krylon UV Resistant Clear Coating Spray] is made for use on artwork that is either outside or near windows. They offer the product in gloss and matte finish as well. Krylon is a very well respected brand, and I believe this will be the easiest and cheapest solution to the problem, albeit not necessarily the best.
Another spray on option would be Premier Imaging Products Print Shield. It is a lacquer based protective spray that is designed for inkjet prints. It also works on photo papers and canvas materials. They specify up to a 2x lightfastness increase, elimination of gloss differential, non yellowing formula, and a reduction in scuffing. The results have been tested by an independent lab (Wilhelm Research) to last up to 166 years with certain Epson papers and regular glass, and over 200 years with UV filtering glass.