The definition is blank.
You've only made the visuals of it, but you need to add some scripts to it. I'd consider searching the source code. 
i need a javascript block to the definition
...for the sake of consistency with other blocks such as "rgba at mouse-pointer", I would go with a list input for the colours. And the list will be r,g,b.
Feel free to ignore the list one.
Also the block should be a stack-shape/command (the horizontal jigsaw), not a reporter (a rounded block).
his first reply!!!11!!!1! yayyyyy!!!1!11!11 /! /ot
This is what I did
And here's my javascript: https://ego-lay-atman-bay.github.io/snap-extensions/block-extensions/categories.js
Hmm.
I would've considered a block like this:
[scratchblocks]
(a new category with name [Category] and rgb (list [63][0][127] @delInput @addInput :: list):: grey)
set [a v] to (a new category with name [Category] and rgb (list [63][0][127] @delInput @addInput :: list):: grey)
[/scratchblocks]
Did you just try to introduce first-class categories into Snap!? 
Is there any peculiar use case it needs to be first class?
add a 'with blocks ⌸ < >' thing
[scratchblocks] (a new category with name [Category] and rgb (list [0][116][143] @delInput@addInput ::list) with blocks ⌸ @delInput @addInput :: grey) (a new block [if %bool %act else %else] with slots (list [2] [5] [5] @delInput @addInput::list) :: grey) [/scratchblocks]
It's already possible in Snap!
(I wonder why block definition tokens in metaprogramming use _ instead of %name. Actually, input names should be defined only once)
I think that's just due to the fact that it would be really awkward to deal with input names being different from what is set in the ring.