Initially, the idea of Mysteries of Light comes from the concept of the decomposition and combination of light and light's different ability to penetrate objects with different light colors. Thus, the core mechanic has built around light combination and penetration in the first demo (the light decomposition is tied with prism in the first demo, and it is not a player's ability). 
Then, after the playtest of the first game demo, we made a big decision of changing Mysteries of Light into a two-player game: because the puzzles in the game require the player to manage lights of different colors, so that having lights of few colors will make the puzzle so easy to solve but having more colors will make the game too complicated. However, if it is a two-player game, the players also need to manage the color use and distribution between two players, which adds tons of fun into the puzzle designs not adding more complexity to the game at the same time. Moreover, we changed that the players can decompose the lights by themselves without the prism because we want to add more freedom and ways for players to manage the color lights in a way they like. In that case, the prism becomes a tool for two players to transport their light colors to help each other solving the puzzle. Finally, we simplified the switches in the game so that they feel more intuitive for players.
In the final version, we mainly focused on the iteration of the tutorial, changing the sequence of introducing different mechanics of the game and how the tutorial are taught to the players.