Answering to myself. Indeed, the palette seems to be lost (all colors are black), but the default palette can be found here: Palette · nesbox/TIC-80 Wiki (github.com) I just need to copy paste the whole string (given on the page), go to the sprite editor, click advanced mode and then click on the "paste" button to use the copied palette hexadecimal string. Now my rectangles are displayed (I dont use sprite for my current project, I only draw pixels and rectangles on the screen)