Cheer. First, head to the GBS docs to read about creating palettes and applying them: https://www.gbstudio.dev/docs/project-editor/scenes#colorizing-a-scene
Second, there are loads of good articles on GB Studio Central (including many of my own). https://gbstudiocentral.com/
Finally, you can check out my GB Studio course if you want some structured lessons. First 6 lessons are free: https://gumpyfunction.itch.io/lets-build-a-platformer