Oh wait actually, it is an object of type Color!
In JavaScript, you can use
return new Color(R, G, B, A);
R, G, and B are 0 - 255 but alpha appears to be 0 - 1.
If you want a good example of how to use this, here are some custom blocks:
Color Blocks Library
Have fun toying around with it in the meantime unless they do plan to open up more of their API and enable flags for most of their features so stuff like this doesn't have to be done the hard way.
The nitty gritty
This is one of the main things that can be annoying in Snap!, since even in Developer Mode a lot of things are hidden to you when you can so easily edit them in XML or JavaScript... (But to do so, you would need to heavily study the source code and properties!!!)
Thus, this feature request sort of coincides with my concept of enabling more things to the user (at least in developer mode) so I don't have to be so hacky. (If developers want a reference, just check out how I do my type [ ] reporter in my XML generator)
The color reporter was made using the generator. To do it manually, %clr is the type you're looking for, and it can also be in multiples with %mult%clr.
If you want to use color as an input, just set it to number and use my color block. I know it isn't a number but it's round and doesn't automatically ringify almost all reporters.
Do note that unfortunately, the pen color cannot be assigned to by the color block unless also doing that by JavaScript. Added this myself. Seems that this is how it's done in the core scripts / source code too.