masonry color assigned by categories

Firstly here is the website I am trying to modify: ThumbZter

Currently the masonry colors are changing depending on the article type (Link, Post, Image and so on)

I want the colors to change depending on Categories not Type

I have a total of 3 categories for the whole web site:
Thumbz Up (I want this to be green)
Thumbz Down (red)
Thumbzter (blue)

No posts are held in dual categories.

I have been struggling with this for too long and need an expert to step in and do this for me. Please help.

Cheers
Fred