Some levels use only 3 or 4 colors. If a specific color is not used then Fiends of that color are not needed for that level.
The colors available in each level were not changed.
Having less colors in the level actually makes the level easier. For example if there are only 3 colors, it is much easier to make longer matches because there are less color variations.
It is now possible to decide if you want to always see all Fiends in the level or if you prefer to hide the Fiends not used in the level you're playing. You can do so in game's Settings Menu > Advanced Settings > Fiend Away Signs.