You all have similar problem to mine.
No idea how to transform into Grayscale 
original colour scale I guess is that: 
and wanted scale looks like this: 
but not this: 
Hello here a solution.
The trick is to transform the color to coordinate. 0 to 255 becomes automatically -1 to 1 in Grasshopper.
So I draw a line on each Gradient, divide each line in N points.
This look like to this.
Then I deconstruct your image. Find the closest color in first gradient. The closest points gives also an index that I use to get the color in second gradient.
“Et voilà”.
The only problem is that the automatic color to point gives [-1 to 1] and the Color from RGB uses [0 to 1] value. It is not very consistant.
Your initial image colors
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