Start a new Scratch project - "File" and then "New"
Delete the cat; right click on it and select "Delete"
Select "Choose new sprite from file" and ...
... from animals select "Butterfly 3"
It looks like this.
Add a script to the butterfly. Start with "When green flag clicked".
Then add "set colour effect by" ...
Now put the "set colour effect" in a "forever" loop.
Change the colour effect to "Whirl".
Change the "0" to be "mouse x"
This is the effect that you will get as you move the mouse about the page.
Now add another script item "set colour effect" to the script and make it change the colour to "mouse y".
Add another "When green flag clicked" to start a new thread. Technical point: each new script for the same sprite is called a thread.
This time add "glide 3 secs to x: y:" and make the x and y positions mouse x and mouse y.
This is the effect that you can make.
Save the project as "Butterfly".
You could extend the project to add a suitable background and another butterfly incoporating different effects.