In fact you can make a category with a translucent or even transparent color by modifying the XML file of the project:
to
, which made:
Have you ever heard about this?
WHAT!?
Hmm. I did this quite a while ago, named $$\colorbox{DarkTurquoise}{\color{DarkTurquoise}!}$$ Window Library, but I never released it.
Clever. I'm obliged to remind y'all that the management is not responsible for bad behavior of projects whose XML you've modified. Please do not report any such bad behavior as a "Snap! bug." 
Hooray! Now we have 4,278,190,080 (4 Billion) more color possibilities for categories!
Some of them are fully transparent!
... except the outlines
And the shadow (or inset?) caused by the text.
No, that's black.
edit: never mind, I guess it's only full black on primitives, but takes the category color in custom categories.
Another edit: It's actually just supposed to be a shadow, but snap creates the shadow by making the category color darker (which removes transparency).
It's a very dark shade of the category color, if you look close enough, it's like a shadow, casted on the top-left. All of the inputs have that effect too.
I'm not trying to start an argument; just bear in mind that that is what I see from the block.
Yeah, I realized that.
It's making every second block of the same category lighter. The reason that the 1st, 3rd, ... blocks appear the same color may be because the block is unaffected due to the fact that there is no effect needed to the block.
O_O
I don't understand. Here is the exact same color, but not transparent:
Of course the zebra-light color should be different. This is a small example (small because Snap! didn't crash) of what I mean about things going wrong when you edit XML files.
I think I remember seeing in the snap source code that the color used for the zebra coloring is set conditionally based on it's parent's color. Plus, it just mixes white with the category color to make it brighter, and since the alpha is set to 0, all that's changed is setting the alpha back to 1 (if alpha is taken into consideration when it comes to mixing colors. I haven't checked though).
Oh, I'm sure it does what it does for some reason. It's a computer! My point is that what it does isn't the desired behavior (and I guess neither is the opacity of the inner colors), but this doesn't count as a bug in Snap!.
Yeah, it's not a bug, as the only way to get it, is by modifying the xml.