Now, I understood, thank you very much for your explanation/feedback!
The background border of levels uses very finely-spaced lines which in practice appear as a different color from the proscribed 2-color palette.
We used no more colours than the given colour pallet for the game rather than the splash screen.
Even the introductory sequence of the game uses additional colors (black and white) beyond the palette constraints.
This does count too?! :)