Forgive me if the answer to this is obvious (I've never been involved in a Gameboy jam before). I understand we have to stick to the original colours and the original resolution. However, given the width/height ratio of the Gameboy screen is different to a computer screen (16:14.4 rather than 16:9 or 4:3) this suggests that the game won't take up the full screen (unless stretched one way more than the other).  Do we have to design the games not to run in full screen or can we make them run on full screen but with the game itself running inside a "game window" which is the correct resolution and uses the correct colours, with a border around it filling in the rest of the space which wouldn't be part of the actual game (and could therefore potentially use different colours to distinguish it)?

This is a super interesting question! Generally, I would advise to have the game run full screen but at the correct gameboy ratio and possibly have plain borders BUT I will allow Super gameboy style border graphics as long as it is clear that it isnt part of the game.

Below are examples of this:

However, I would also add that these background must also be four colours (but they can be a different four colours to your game)

That's great. Thanks for the quick reply and the very useful response.

No problem. If you have any more questions or issues, ping me on twitter!