Help with my color picker

I know what the issue is but i do not know how to go about it, my x coordinate isnt relative to the image.
How would i for example turn 70% into 30% and 40% into 60%?
Ive attached a video below.

Can you show the code? It might be able to help anyone else who helps.

100-70 = 30
100-60 = 40

i am not sure if i understanded right

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