I really enjoyed this! I liked that you had the colours arranged in the corresponding order of the keys (made remembering which key press to use much more intuitive).
2 main pieces of feedback:
1) I would recommend a brief pause after the player successfully solves a 'Simon says'. As it currently stands, the moment the last input is received the next one starts up, making it quite tough first time round to catch all the requested colours.
2) As a fellow Godot user, I understand the struggle of in-game camera resolution vs. export resolution. As it currently is, the playspace on my screen is small box in the top left, with the rest of the screen being grey. Look into the project settings menu and have a play around with the Display Scale settings, so that your game space fills the whole screen when played at different monitor resolutions!