Do Miniature Paints Fade?
This is a question I had been pondering for a while - are any of the major paint manufacturers throwing out paints that do not keep their colour? I had just got a nice glass and mirror display cabinet with down lighting and had put it directly facing the window for maximum light and therefore maximum effect. Before, I had kept everything in storage boxes, but now I wanted to enjoy my work every day. My fear was that over the decades, the models would lose their lustre, change altogether, or be impossible to colour match when I want to add to them in the future.
I started a few forum posts in various places to see if anyone who has had models in cases for long times had noticed any fading. Nobody could give a definitive yes/no answer, but the general attitude was 'probably not'. I therefore thought it would be a good idea to set up a little experiment to see just what is the truth.
Unscientific Plan
From a science perspective, there are a million and one issues with this. The idea was to do a crude, easy test that should inspire people to examine the results in more detail, with better tests in the future if needed. The premise was simple, paint some swatches, have half the swatch exposed to light and half hidden from light in darkness.
Gathering Materials
Step one was to get the paints I needed...

different paint ranges that have roughly equivalent colours
I wanted to test the major primary colours, white and green for each of the major ranges - Vallejo (model and game), Citadel (standard, foundation, wash), Coat D'arms (the old citadel paints), Reaper (just master series) and Privateer Press. At the time, these represented the vast, vast majority of wargamer paints. I excluded Tamiya, other Repear ranges, any non-acrylics, and any artist acrylics simply due to cost and complications.
Varnish was another issue - would it increase the rate of fading, decrease it, or have no effect? I used Winsor and Newton Artist's Matt Varnish as that is the one I usually use on my models in some form.
Next, I printed a sheet of A4 paper with a bunch of swatches on them. I then took a cereal box and marked and cut windows for half of each of the swatches.

paint swatches on the day of creation
Counting from top left to right, then next line left to right, etc.
1. GW - Skull White
2. Coat d'Arms - White
3. P3 - Menoth White Highlight
4. Vallejo Game Color - Dead White
5. Vallejo Model Color - White
6. Reaper Master Series - Pure White
7. GW - Blood Red + Matt Varnish
8. GW Wash - Baal Red + Matt Varnish
9. GW Foundation - Mechrite Red + Matt Varnish
10. Coat d'Arms - Blood Red + Matt Varnish
11. P3 - Skorne Red + Matt Varnish
12. GW - Golden Yellow
13. GW Wash - Gryphonne Sepia
14. GW Foundation - Iyanden Darksun
15. Coat d'Arms - Golden Yellow
16. P3 - Sulfuric Yellow
17. Vallejo Game Color - Gold Yellow
18. Vallejo Model Color - Golden Yellow
19. Reaper Master Series - Marigold Yellow
20. Vallejo Game Color - Bloody Red + Matt Varnish
21. Vallejo Model Color - Vermilion + Matt Varnish
22. Reaper Master Series - Blood Red + Matt Varnish
23. GW - Goblin Green
24. GW Wash - Thraka Green
25. GW Foundation - Knarloc Green
26. Coat d'Arms - Goblin Green
27. P3 - Iosan Green
28. Vallejo Game Color - Goblin Green
29. Vallejo Model Color - German Cam. Bright Green
30. Reaper Master Series - Field Green
31. GW - Ultramarine Blue + Matt Varnish
32. GW Wash - Asurmen Blue + Matt Varnish
33. GW Foundation - Mordian Blue + Matt Varnish
34. GW - Blood Red
35. GW Wash - Baal Red
36. GW Foundation - Mechrite Red
37. Coat d'Arms - Blood Red
38. P3 - Skorne Red
39. Vallejo Game Color - Bloody Red
40. Vallejo Model Color - Vermillion
41. Reaper Master Series - Blood Red
42. Coat d'Arms - Marine Blue + Matt Varnish
43. P3 - Arcane Blue + Matt Varnish
44. Vallejo Game Color - Ultramarine Blue + Matt Varnish
45. GW - Ultramarine Blue
46. GW Wash - Asurmen Blue
47. GW Foundation - Mordian Blue
48. Coat d'Arms - Marine Blue
49. P3 - Arcane Blue (this is not very close to the others but should be close enough to not impact the results too much)
50. Vallejo Game Color - Ultramarine Blue
51. Vallejo Model Color - Ultramarine + Matt Varnish
52. Reaper Master Series - Ultramarine Highlight
53. Vallejo Model Color - Ultramarine
54. Reaper Master Series - Ultramarine Highlight + Matt Varnish
55. GW Wash - Gryphonne Sepia + Matt Varnish

The completed test box
The next job was to stick the box to the window for 3 months at a time, over the course of a whole year. Thanks to my wife who let me do this! The window was south facing so had direct sunlight for the bulk of each day (well as much as you can get in London anyhow).

3 Months Later

3 months of exposure to the sun
6 Months Later

6 months of exposure to the sun
9 Months Later

9 months of exposure to the sun
12 Months Later

12 months of exposure to the sun
And a high contrast version to make it easier to see differences between swatches exposed to light (top half), and kept in darkness (bottom half):

12 months of exposure in high contrast
All coloured ink on the window facing parts of the cereal box had faded to nothing after 12 months, so the level of light exposure was satisfactory for my simple test. Seeing the box fade as time went on made me very interested to see what was happening to the paints, but I only took them off once every 3 months, took them out of the box, put them in the scanner (the same scanner was used for all 5 scans), and then straight back in the box and up on the window again.
As we can see in the 12 month version, and even more clearly in the high contrast version, almost all the paints held up with no apparent change. A couple of the whites are fractionally different, but that could be put down to exposure to dust as much as exposure to light.
The big difference was in the Citadel Washes. Gryphonne Sepia was the first to start to fade, but clear fading can be seen on all wash colours, with and without varnish except Asurmen Blue which darkened slightly, though did suffer a small change. Varnish did not make any difference - they still faded at the same rate regardless of vanish.
My apologies for the poor quality of the scans - the swatch paper was glued on to a cardboard backing which warped as time went on, causing it to bend slightly in the scanner. In the interests of consistent scanning, I did not change the scanning methods so that direct comparisons between images remained feasible as it was the most controlled environment I could create (without expending any real effort!)
Conclusion
So to conclude, be wary of citadel washes if you want consistent colour over a long term if your models are stored in direct sunlight. Use them for shading by all means, but painting a model entirely in them might give you trouble trying to colour match in the future.
I was very relieved to see that none of the actual standard acrylics from the major ranges fade to any significant level, and can keep my models in my cabinet with the lights shining down.
I hope you found this article somewhat interesting - now I am the 'Crazy cereal box in his window' guy around here...