It will be only these two colors + their alpha channels (transparency can only be implemented at the code/engine level)
transparency can only be implemented at the code/engine level
This rule seems fairly arbitrary, difficult to enforce, and favors technical artists/more sophisticated engines (it's also not actually written in the gamejam rules). Most art applications will allow you to use a gradient to apply transparency as a mask (which is fast), or you could write a shader to get the exact same effect (which is arguably slower and at the very least much more annoying to do on a case-by-case basis).
I guess I'm mostly just confused about the purpose of such a rule, and about the spirit of this gamejam in particular (for my part, I'm using a black and white palette and changing material transparency in engine in most cases, but I'm also using vector art for the backgrounds and there are some places where having a white line over a black background subtly trail off to a lighter gray by using a transparency gradient is a nice little flourish).
Well, the point about the alpha channel was made to allow effects such as fade-out (not really post-processing). But in order for it not to allow the gradient, a line about the engine/code was added. A lot of game jams involve some restrictions so that people get out of their comfort zone. In our jam, you can create a game using only two colors, with some assumptions, and the shades of these colors are considered separate colors.
