Currently, the negative graphic effect, when used at a value of 100, makes it not actual negative, but rather an ugly negative effect:
Now, I created a custom block that allows you to change how the negative effect works:
(to use it, drag and drop the block to the Snap! editor)
Run the block with JS enabled, and use the negative effect and you'll get the following result:
But this is slower and time-consuming. My JS function seems to allow inverting colors faster than usual. Also, I want to make this as a graphic effect, not a costume.