It would be much better not to use the word "tool"in this way. In Gimp, a
is an element of the user interface. When the user applies a tool, the end
result is to perform a series of operations on the image. Thus, a tool is a
user interface element that allows the user to interactively specify a
of operations. What you are talking about storing as part of the image are
color operation layers, not tool layers.
The flip side is that the way a tool works can be changed without violating
backward compatibility. If you can find a way to make the Hue/Saturation
work better, there's no strong reason it couldn't be put into Gimp.
layer modes that are stored in XCF files is more problematic.
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