I just found out the colours are hardcoded.
I see two colours, because there is only 1 category colour defined (of 3 used categories, but since 2 are white, I think the code only shows white only once.)
I see this being used.
<div class="fc-event-inner fc-event-skin" style="background-color: rgb(229, 69, 74); border-color: rgb(229, 69, 74); color: rgb(255, 255, 255);">
Isn't it an idea to give each item a unique class like post_class() ?
Instead of just "fc-event-inner fc-event-skin" because this doesn't identify an event uniquely.
In my case this event would then have 3 categories;
<div class="fc-event-inner fc-event-skin category-slug1 category-slug2 category-slug3"
These can all be identified uniquely through a stylesheet.
I think it's a good idea to include tags here as well.
The other idea is to only define a colour when the value is set. So upon initialization, the category (bg) colour should read none, instead of white. If the value is none, it shouldn't be hardcoded.
Anyway that's my 0,02...
If anyone knows a way to hook in a custom piece of code for it, I'm interested.