The colors should be used with full alpha, if possible, but one could use other tricks for selectors (e.g. putting the selector color behind the item and the frame in front of it). Trying to make a proper UI within such strict limitations is part of the challenge :)

That said, the goal of this casual jam isn't to frustrate the developers, so if you can't find a way out, or the workaround you have is too complex, feel free to use transparency as a fallback solution - but try to limit it only to UI elements.