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 and their power can't be charged.
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 the game's Settings Menu > Advanced Settings > Fiend Away Signs.
This feature was introduced to help players make team selecting and Fiend upgrading decisions before the level starts. For example, if you know in advance that the red colour will not be used in the level, you may want to avoid upgrading your red Fiend before the level and upgrade a Fiend of a different color instead.