I think the problem was that the plugins were loaded in the wrong order. Tried with vanilla Essentials and with some plugins like: Following Pokemon EX, Gen 8 Pack, Essentials Deluxe and it should works now. Basically these plugins override the same method so the last plugin that is compiled...
Sorry if I ask but are you sure you installed it correctly?
When compiling you should see: -> Loaded plugin: Pokemon Color Variants (ver. 1.0.0).
This may explain the no variants spawning and the error shown in the picture.
About the error: make sure to follow the exact syntax writed on Github...
Since essentials uses the same icon for normal and shiny pokémon I thought it was better to use them aswell for a hue variant.
Probably you are trying to use specific and no specific hues simultaneously.
If SPECIFIC_HUE_ONLY is set to true you have to specify the hue variants for each pokémon...
Pokémon Color Variants
Forge your rainbow team!
Overview
More than 700 variants for each pokémon
Encount the pokémon variants in the wild or obtain them through breeding
Bring more color to trainers team
Create unique and wonderful variants to reward the player
Customize the plugin to fit...