Hello GDevelopers:) I have a question: What is the “ShiftIndex” in pixels editor? what is this do? 
I think you mean Piskel. That’s a separate tool bundled with GDevelop, you’re best off looking at the documentation for that system.
From a quick google, it looks like it was implemented a while back, here’s the feature discussion on Github from the person who built it: Colour Index shift brush - similar to lighten/brighten brush, but without the problems · Issue #863 · piskelapp/piskel · GitHub