Hello, again;
> Is it possible to
> - Add more colors to a sprite?
Not yet, since the tool is currently focusing on the standard kernel; once that part is finished, it may be possible to look at other kernels.
> - Add more sprites?
Again, not yet, until it is modified to use the other kernels.
> - Change background colors
This one isn't implemented yet, but since it is compatible with the standard kernel, it should be implemented "soonish": https://github.com/haroldo-ok/vcs-game-maker/issues/37